Preheat oven to 400 degrees. Lightly coat a 9 x 13 inch casserole dish with nonstick spray.: Warmth in the oven readies the environment so the top browns as the filling heats through, producing a pleasing golden surface. When you slide the dish in, you should hear a gentle intake of warmth, and that initial heat jumpstarts the bubbling along the edges. A common mistake is skipping the preheat, which can result in uneven baking and a soggy top, so always wait until the oven reaches temperature.
In medium skillet, heat oil on medium heat. Add onion, garlic, celery, and carrot. Cook until onions are translucent. The carrots may still be crunchy. Add peas, salt, turkey, gravy, rosemary, oregano, and sage. Bring to a boil and simmer for 1 minute. Pour into prepared casserole dish.: As the extra virgin olive oil warms, it will shimmer faintly and release a soft olive perfume, which is your cue to add the aromatics. The onion will soften and become sweet, while the sizzling of the garlic and vegetables creates a comforting sound. When you add the gravy and shredded turkey , the mixture should look glossy and slightly thickened, coating the vegetables. Letting it simmer for about a minute helps flavors meld and reduces any rawness from the aromatics. Watch carefully to avoid over reducing, which can dry the filling. A common pitfall is cooking on too high a heat, causing the garlic to bitter; medium is steady and reliable here.
In a separate bowl, combine both potatoes, egg whites, and parsley. Mix well. Drop by spoon fulls on top of turkey mix. Using a spoon, spread as evenly as possible over the turkey. Sprinkle with parmesan.: When you mash the potatoes and sweet potatoes , notice their silky texture and warm steam, which helps them meld with the egg whites and parsley . The mixture should be cohesive but not gluey, and dropping spoonfuls creates rustic peaks that brown attractively. Spreading gently ensures the filling is sealed beneath, preventing drying. Sprinkle the parmesan cheese evenly to encourage a savory, browned crust. Overworking the mash can make it gummy, so mix minimally for the best mouthfeel.
Bake for 20 minutes or until top begins to brown. Let set for 5 minutes before serving.: As the pie bakes, you will see the edges bubble and a faint golden color develop on the potato peaks. The aroma will shift toward roasted, herbed notes, and you may hear a soft popping at the edges where gravy warms. Allowing it to rest for a few minutes lets the filling settle, making it easier to portion and preventing a runny serving. Cutting too soon often causes a loose slice, so patience pays off here.