Adjust oven rack to middle and preheat to 425°F.: As you prepare the other elements, the kitchen fills with the scent of melted butter and herbs, and the preheated oven ensures a quick, even bake that sets the top without overcooking the pasta. This high heat gives a slight golden finish, signaling the sauce has caramelized a touch, which rounds flavors. A common misstep is skipping preheating, which leads to uneven bake and a soggy finish.
In a large (3.5 quart) oven-safe pan over medium-high heat, heat 1 tbsp oil from the sun-dried tomatoes. Add mushrooms and cook for 2–3 minutes. Then add sun-dried tomatoes and cook another 2–3 minutes. Add minced garlic and cook for 30 seconds. Transfer everything to a separate plate.: The pan will sing as the mushrooms release steam and then begin to brown, producing an earthy aroma. Adding the sun dried tomatoes with their oil encourages caramelization and a concentrated sweet tang, while the garlic at the end becomes fragrant without burning. Remove to a plate so you preserve fond, which is key for the sauce base. If you overcrowd the pan, the mushrooms will steam rather than brown, losing flavor development.
In the same pan, melt remaining 4 tbsp butter. Sprinkle flour over the melted butter and whisk briskly until smooth. Continue cooking and whisking constantly for 1–2 minutes. Stir in Italian seasoning, paprika, red pepper flakes (if using), plus salt and pepper to taste.: The whistle and soft bubbling as you cook the roux indicate the raw flour taste is dissipating, while whisking keeps lumps from forming. When the roux is pale and nutty, the spices bloom in the fat releasing aromas that hint at the finished sauce. Overcooking to a dark color will change the flavor away from creamy to toasted, which is not desired here.
While whisking constantly, very gradually pour in the milk and chicken broth until the mixture is smooth. Bring to a boil, then reduce heat to medium and simmer until slightly thickened, about 3–5 minutes. Reduce heat to low and gradually add 2 tablespoons of Parmesan cheese at a time, stirring until melted.: The sauce should move from chalky to glossy as liquid is incorporated, and steady whisking prevents lumps while coaxing creaminess from the starch. When it reaches a gentle simmer, you will see it thicken and coat the back of a spoon. Adding cheese slowly ensures it melts evenly and the texture stays silky. A frequent mistake is dumping cold milk or broth in too fast, which causes clumps and an uneven sauce.
Return the cooked veggies (along with accumulated liquid) to the pan. Add rotisserie chicken, tortellini, and chopped spinach. Gently stir to combine, ensuring the tortellini is submerged in the liquid as much as possible and smooth into an even layer.: The return of the browned bits and vegetables deepens the sauce color and aroma. Stirring gently distributes heat and prevents tearing the filled tortellini , while submerging the pasta lets it hydrate so it finishes tender in the oven. Too vigorous stirring can break the pasta and create a pasty texture.
Place pan in oven and bake for 8 minutes. Remove from oven, stir everything well, and press into an even layer. Sprinkle the remaining 3 tablespoons of Parmesan over the top and bake for an additional 3 minutes.: The oven warms the center through, and you will notice the edges begin to bubble softly, releasing a fragrant steam. This initial bake allows the tortellini to finish cooking gently while the sauce sets beneath the surface. Avoid leaving it in much longer at this stage as the pasta can overcook and absorb too much sauce.
Remove from oven, season with additional salt and pepper if needed, and garnish with fresh herbs if desired. Serve immediately for best results, as the tortellini will continue to absorb the sauce if left to sit.: Stirring after the first bake melds pockets of sauce and ensures even distribution before the final cheese topping melts into a slightly golden finish. The final short bake produces a pleasing top texture without drying the interior. A mistake to avoid is skipping the stir, which can leave uneven sauce pockets and a drier top.
Remove from oven, season with additional salt and pepper if needed, and garnish with fresh herbs if desired. Serve immediately for best results, as the tortellini will continue to absorb the sauce if left to sit: When you slice into the bubbling pan, you should see glossy sauce and steam carrying the roasted aroma of mushrooms and tomatoes . Serving right away preserves the creamy texture and prevents the tortellini from soaking up too much sauce, which keeps each bite luscious. Letting it sit too long will lead to a thicker, drier casserole style, so reheat with a splash of broth if serving later.