Place the potatoes in a medium saucepan, cover with water, 1 tablespoon of salt as you would pasta water, and bring to a boil for about 16 to 18 minutes, or until tender.: You will hear a soft rolling boil and see the water bubbling steadily, with the diced sweet potatoes bobbing gently; this is the sign they are cooking through. The kitchen will start to smell faintly sweet as the starches release. Use a fork to test tenderness, aiming for pieces that yield without collapsing, which ensures a silky mash rather than a gluey one. The reason for generous salted water is to season the potatoes from within, which deepens the final flavor. A common mistake is overboiling until the potatoes fall apart, resulting in a waterlogged mash; keep an eye on the time and test early to prevent that.
Drain, and return to saucepan, add 1/4 teaspoon salt, sour cream and mash with a potato masher, keep covered.: After draining, you will notice a warm, concentrated aroma from the sweet potatoes . Adding the measured salt and the sour cream creates a creamy, tangy mash that should be smooth but still have slight texture. Use a masher and press firmly for a few pulses rather than overworking, as vigorous beating can make the mash gluey. Keep the pot covered to retain heat and moisture while you complete the filling, which also allows the flavors to meld. People sometimes skip the dairy addition or add too much liquid, producing a runny topping, so add the sour cream gradually and aim for a scoopable consistency.
Preheat oven to 375 F with a rack in the center.: You will sense warm air in the oven when it reaches temperature, which is essential so the filling heats through evenly and the top gains color without burning. Preheating ensures consistent bake time and prevents undercooked interiors. Set your rack to the center to allow balanced browning. A frequent oversight is placing the dish too close to the broiler, which can darken the topping before the filling is hot; center placement prevents that.
Heat a large skillet and add oil over medium heat. Add the shallots, parsley, thyme and sage and cook until almost soft about 2 to 3 minutes.: The oil should shimmer slightly when hot but not smoke; this indicates it's ready for aromatics. Heating the oil slowly helps release flavors without burning. If the oil smokes, lower the heat and start again, because burnt oil will impart a bitter taste. Many cooks either overheat the oil or keep it too cool; aim for a gentle sizzle when you add the next ingredients.
Add the celery and cook until the vegetables are soft, about 4 to 5 minutes.: As these aromatics hit the pan, you will hear a soft sizzle and smell an instant lift of herby, oniony fragrances. The shallot softens and the herbs bloom, releasing essential oils that form the aromatic backbone of the filling. Cook just until they are softened but not browned to keep a fresh herb character. Overcooking here can yield a flat, muted flavor, so watch for translucence and a fragrant aroma.
Add the frozen mixed vegetables, turkey, 2 cups turkey broth, 1/4 teaspoon salt and black pepper and bring to a boil. Stir and simmer over medium-low heat 8 to 10 minutes, stirring until the flavors meld.: The celery will soften and release a vegetal sweetness, and the pan may pick up a fond, tiny brown bits that intensify flavor. You should be able to smell a mellow, savory base forming; stirring occasionally prevents sticking. The purpose of softening is to integrate texture and allow the celery to contribute body to the sauce. Rushing this step often leaves celery too crunchy, breaking the harmony of textures in the finished pie.
Meanwhile combine 2/3 cup turkey broth with the cornstarch and mix well to dissolve. Add to the turkey and cook over medium-low heat, stirring until thickened, about 3 to 4 minutes.: When the frozen peas and carrots hit the hot pan, you'll hear a quick hiss and see steam rise as they warm. Adding the chopped turkey and the measured broth transforms the skillet into a simmering stew, and the aromas will deepen into savory, herbaceous notes. Bringing to a boil helps integrate flavors quickly, and then lowering to a simmer encourages gentle melding. One pitfall is boiling too hard, which can break down the vegetables and make the sauce thin, so reduce heat once it reaches a boil.
Remove from heat and transfer to an oven safe, deep 9 x 9-inch baking dish or deep round pie dish. Or make them in 4 individual broiler-safe 8 ounce ramekins.: During the simmer, the broth reduces slightly, and the filling will smell rich and cohesive, with the herbs and shallot blended into the liquid. Gentle bubbling is the goal, you should see small, steady bubbles and occasional steam. This slow meld deepens the flavor and softens all components to the right tenderness. Over simmering can dry the mixture and concentrate salt, so follow the time range and adjust heat if the liquid drops too fast.
Place the mashed sweet potatoes over the turkey filling.: Mixing cornstarch into cold broth prevents lumps and ensures a smooth slurry that will thicken evenly when added. The slurry should look silky and free of grit; if you see clumps, whisk them out now. The purpose is to create a stable thickener that gives the filling body without floury taste. A common mistake is adding cornstarch directly to hot liquid, which causes clumps and uneven thickening.
Bake on a sheet pan 20 minutes, until heated through then finish under a broiler 2-3 minutes for extra color.: As the slurry warms, the sauce will change from glossy and thin to clingy and slightly translucent, coating the spoon and turkey pieces. You'll hear gentle bubbling and see the sauce cling to the pan, which is the visual cue for readiness. This thickening step ensures the filling does not separate or become soup when baked under the mash. Avoid boiling vigorously, which can break down the thickener and thin the sauce, and stir to prevent scorching on the pan bottom.
Remove from heat and transfer to an oven safe, deep 9 x 9-inch baking dish or deep round pie dish. Or make them in 4 individual broiler-safe 8 ounce ramekins: Transferring the filling allows you to assemble confidently, and you will feel the heat radiating from the dish. Spread evenly so the mash will sit level on top; uneven filling can lead to uneven browning. Choose a dish that is deep enough to hold the filling without overflow. A misstep is using a shallow dish that bubbles over during baking, so err on the deeper side if unsure.
Place the mashed sweet potatoes over the turkey filling: The warm mash should be spooned or piped over the hot filling, creating a smooth or textured top depending on your preference. The contrast of the hot filling and the creamy topping creates a brief steam burst when baked, which helps with melding. Smooth the top with a spatula for even browning, or rough peaks for more surface color. If the mash is too loose, it can spread and thin, so ensure a scoopable consistency before topping.
Bake on a sheet pan 20 minutes, until heated through then finish under a broiler 2-3 minutes for extra color: During the bake the filling will bubble at the edges and the mash will set; you will smell the herbs and the kitchen will feel cozy. Finishing under the broiler for 2 to 3 minutes gives the sweet potato peaks a caramelized, golden hue, adding texture and aroma. Watch closely under the broiler to avoid charring, since color can move quickly. A common error is skipping the sheet pan, which can make cleanup difficult if the filling bubbles over.