Saute onion and mushrooms in butter until lightly browned. Remove from pan. Add milk to same skillet. Add bouillon and cornstarch. Mix well. Cook over medium heat, stirring often, until sauce thickens. Season with salt and pepper. Add pasta, vegetable mixture, chicken and peas. Mix to combine.
Place mixture in cassserole. Sprinkle with parmesan cheese. Bake in 350 oven for 25-30 minutes.