Preheat oven to 350 degrees F (175 degrees C). Prepare a 13x9 inch baking dish with nonstick cooking spray.
Bring a large pot of lightly salted water to a boil. Cook rotini at a boil until tender yet firm to the bite, about 8 minutes; drain. Pour rotini into prepared baking dish.
Melt butter in a heavy saucepan over medium heat. Cook and stir onions, celery, and bell pepper in melted butter until softened, about 5 minutes.
Stir milk, cream of mushroom soup, poultry seasoning, salt, and black pepper into onion mixture. Reduce heat to medium low and bring to a simmer; about 5 minutes. Add chicken and 1/2 cup Cheddar cheese to milk mixture; remove immediately from heat and pour over rotini. Top pasta and sauce mixture with remaining Cheddar cheese.
Bake in preheated oven until cheese is melted and beginning to brown, about 25 minutes.
Fast and Easy Chicken Tetrazzini · Dinner Match Lab | Dinner Match Lab