Preheat oven to 350°
Cut chicken into 1" pieces.
Cook chicken breasts in a skillet in 1 tablespoon of butter over medium heat. Cook 6 to 8 minutes, until light brown and set aside in a casserole dish.
Cook and drain pasta as directed on package while chicken is cooking and drain. Place in a bowl and set aside.
Saute onions, mushrooms and garlic in skillet with 1 tablespoon butter for 3-4 minutes until mushrooms are browned. Add mushrooms to the casserole dish.
Saute spinach over medium-low in the same skillet for about one minute and add to casserole dish. Add another teaspoon of butter to skillet if needed.
Add chopped tomatoes to the casserole.
In a small sauce pan, melt remaining 3 tablespoons butter over low heat.
Add flour to butter and whisk.
Add half and half to flour mixture and continue to whisk.
Cook about 1 -2 minutes or until thickened. If the sauce is too thin, add a little more flour.
Add the Mozzarella cheese and salt to sauce and mix together.
Pour cheese sauce into cooked pasta and stir. Then add pasta and sauce into the casserole dish.
Sprinkle with bread crumbs and bake 10-15 minutes.