Bring a pot of lightly salted water to a boil. Preheat oven to 375 degrees F. Spray a 9 x 13″ baking dish with cooking spray, and set aside.
Cook pasta according to package directions, and drain.
While pasta cooks, melt butter in a large skillet over medium high heat. Add onion and garlic, and saute for about 3 minutes.
Add chicken, peas and carrots, poultry seasoning, salt, and pepper. Stir fry until cooked through, about 5-7 minutes.
Add flour, and stir to coat the chicken. Add about 1/2 cup milk, and stir until smooth. Slowly pour in remaining milk, stirring constantly, to keep mixture smooth. Bring to a simmer.
Stir in cheese, and continue to simmer until all cheese is melted. Turn off heat. Stir in cooked pasta until combined well, and transfer to prepared baking dish.
For the Topping
Melt butter in a microwave-safe dish in the microwave (or on the stove if you prefer). Stir breadcrumbs into the melted butter.
Sprinkle 1/2 cup cheese over the pasta in the baking dish, and sprinkle the breadcrumbs over the cheese. Bake in preheated oven for 20 minutes or until top is golden brown and edges are bubbly. Serve right away.