Chicken tetrazzini bakes tender pasta and chicken in a creamy Parmesan sauce for a satisfying family casserole. This protein-rich dish is perfect for using leftover chicken and makes great freezer meals.
Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish with a thin layer of butter or nonstick spray.
In a large skillet over medium heat, melt 2 tbsp butter and olive oil. Add chopped onion and mushrooms, sautéing until soft and lightly browned (about 6 minutes).
Stir in flour and cook, whisking constantly for 2 minutes to eliminate raw taste. Gradually pour in milk and chicken broth, continuing to whisk until the mixture thickens and bubbles gently.
Reduce heat to low. Add salt, black pepper, garlic powder, thyme, and paprika. Blend in 2/3 cup Parmesan, stirring until melted. Turn off heat.
In a large bowl, combine cooked spaghetti, shredded chicken, sautéed peas, and the creamy sauce. Toss to coat well, then transfer to prepared baking dish and spread evenly.
Dot the top with the remaining 2 tbsp butter and sprinkle with 1/3 cup Parmesan. Bake uncovered for 25–30 minutes, until bubbling and lightly golden on top.
Let rest 10 minutes before serving for best texture and to prevent breakage of the casserole slices.