Preheat oven to 350F. Cut the tops off of the peppers and set them aside (throwing away the stem itself).
Wearing rubber/plastic gloves, take a knife and chop off the top of the peppers and run the knife on the top about 1/4” from the tip to the top of the pepper. You don’t want to split them in half. You want the pepper whole but have the slit so you can stuff it. Remove the rib and seeds; discard.
In a separate bowl, add the sausages, onion, green and hot pepper tops, spices, ¾ cup of the mozzarella, egg, bread crumbs (start with a 1/2 cup), and 1/2 cup marinara sauce. Gently mix to incorporate. If the mixture is too wet add the rest of the bread crumbs. You want the mixture more on the moist side. It should be semi-loose in structure.
Gently open up the pepper and stuff it with the meat mixture slightly mounding. Place each filled pepper in a 9×13” baking dish. Add the marinara sauce over top of the peppers – at a bare minimum, you want to use at least 1 1/2 cups. You can add more if you want more sauce.
Cover with foil and bake for 40-55 minutes or when an inserted meat thermometer into the sausage reads 160-165F. Remove foil and top with remaining cheese.
Bake uncovered for 5-10 minutes or until the cheese is melted.