Preheat oven to 375 degrees F. Take chicken breasts and lay under the parchment paper or plastic wrap on a smooth clean surface or cutting board, and pound with meat pounder until flat.
Preheat skillet on medium high with butter. Thaw out frozen spinach and drain. Finely chop the garlic, onion and mushrooms then place all the ingredients into skillet, saute for about 5-7 min.
Take salt, pepper, and rosemary and season the chicken. Put 1 cup of each cheese, and all of the spinach mixture onto the chicken breasts. Then roll the chicken breasts very tightly removing the parchment paper or plastic wrap and place the toothpicks into the chicken.
Place chicken into a medium sized casserole dish or deep pan with toothpicks still inserted. Bake chicken for 1 hour.
Take out of the oven and add spaghetti sauce and the reminder of the cheese for about another 30 minutes to 1 hour or until chicken is cooked thoroughly and cheese is melted/golden brown. Remove toothpicks and let cool for about 10 minutes. Slice or serve whole.