You may make more than one package of sausages as long as the sausages will fit in the casserole; the amount of marinara is generous.
Preheat oven to 400 degrees F. Spray a casserole dish.
Using the tip of a very sharp knife, slice each Italian sausage from one end to the other along the length. Spread apart to form a pocket, then fill with a string cheese. Place in casserole, cheese up.
Cover with the marinara sauce. Cover with foil and bake for 30 minutes. Uncover and bake another 10 to 15 minutes until sausage is cooked through, cheese is melted and marinara is bubbly.
Remove from oven and let sit for about 10 minutes before serving.