Step 1: Preheat your oven to 375°F (190°C). Lightly grease a baking dish large enough to hold the banana peppers.
Step 2: Prepare the banana peppers: Wash and pat them dry. Carefully slice each pepper lengthwise down one side, from stem to tip, and gently remove the seeds and membranes. You can use a small spoon or your fingers. Be careful not to tear the peppers. If you prefer more heat, leave some seeds in.
Step 3: Prepare the filling: Heat olive oil in a large skillet over medium heat. Add the Italian sausage, breaking it up with a spoon, and cook until browned and no longer pink. Drain any excess fat.
Step 4: Add the chopped onion to the skillet with the sausage and cook until softened, about 5 minutes. Stir in the minced garlic and cook for another minute until fragrant.
Step 5: Remove the skillet from the heat. In a large bowl, combine the cooked sausage mixture, 1/2 cup marinara sauce, breadcrumbs, 1/4 cup Parmesan cheese, beaten egg, 1/2 cup mozzarella cheese, and fresh parsley. Season with salt and pepper to taste. Mix well until everything is thoroughly combined.
Step 6: Carefully stuff each banana pepper with the sausage and cheese filling. Don't overstuff; make sure the filling is packed snugly. Arrange the stuffed peppers in the prepared baking dish.
Step 7: Spoon a little extra marinara sauce over each stuffed pepper, then sprinkle with additional mozzarella and Parmesan cheese.
Step 8: Bake for 25-35 minutes, or until the peppers are tender, the filling is heated through, and the cheese topping is melted and bubbly golden brown. Let cool slightly before serving. Enjoy your delicious banana pepper recipe!