Slice your Italian sausages down lengthwise.
Stuff your sausages with cheese.
Try to seal your sausage as much as you can.
Place all of your stuffed sausages into a 9x13 pan.
If desried, cook your sausage and onion in a large skillet on medium to medium high heat. Cook as long as desired.
Top your sausages and peppers with the marinara sauce. If you are meal prepping and storing in the fridge, I would stop here. I feel like the cheese might burn before the sausages get hot.
Top with shredded cheese and Italian seasoning. Bake at 375 degrees for 30-35 minutes, until hot and bubbly.
Serve hot with a green veggie on the side.