Preheat oven to 350
Grease a 9x13 baking dish and set aside.
In saute pan, over medium heat, cook chicken breasts about 4 minutes per side. Generously salt and pepper each side.
Once cooked through, chop into bite-sized pieces and set aside.
Cook spaghetti until al dente, according to package instructions.
Drain and set aside.
Melt 1 tablespoon butter in a frying pan. Add onion and saute until clear. Add garlic and mushrooms. Cook a few minutes, until tender
Remove from pan, set aside.
Melt remaining 4 tablespoons butter in pan.
Whisk flour into butter for 2-3 minutes.
Whisk in the milk. Add salt and pepper.
Cook until thickened. Add 1 cup of mozzarella cheese and stir until melted and combined.
Add mushroom mixture and chicken and stir to combine.
In a greased 2-quart baking dish, arrange a layer of spaghetti, then a layer of creamed chicken, and a sprinkle of paprika.
Repeat layering until dish is full.
Sprinkle with grated parmesan and remaining mozzarella cheeses.
Bake 35-45 minutes minutes or until bubbly.