In a 12 inch, oven-safe skillet, crumble and cook sausage over medium high heat until no longer pink. With a slotted spoon, transfer sausage to a paper towel lined plate to drain.
Add onion to the skillet and saute until soft; about 3-4 minutes. Add garlic and cook, stirring, for about 30 seconds. Turn heat to medium low. Pour about 1/2 cup spaghetti sauce into a measuring cup, and pour the remainder into the skillet. Stir together and cook for a few minutes until sauce starts to bubble. Add half of the sausage to the skillet. Turn off heat.
Butterfly chicken breasts by slicing into the long side of the breast, stopping just about 1/4 of an inch from the opposite side. Stir together remaining sausage with about 1 1/3 cups of shredded Mozzarella. Stuff each breast with a third of the sausage mixture and secure the sides together with baking twine or toothpicks. Place seam side down in the skillet with the sauce. Pour remaining sauce over chicken and brush evenly. Sprinkle with basil, parsley and oregano, then top with remaining shredded Mozzarella.
Stick a tooth pick into each breast (should stick out about an inch and a half so that the foil won't touch the cheese). Cover with foil (you might need two sheets to cover the whole pan).
Bake at 400 degrees F for 25 minutes. Uncover and bake for an additional 10 minutes or until cheese is melted and chicken is cooked through.
I love to serve this with garlic bread and a green salad.