Preheat the oven to 375°F (190°C) and lightly grease a 9 × 13-inch baking dish.
Wash the banana peppers and slice each one lengthwise.
Remove the seeds and inner membranes.
Spread approximately 1 1/2 cups marinara sauce across the bottom of the prepared baking dish.
Arrange the banana pepper halves cut-side up over the sauce.
Heat the olive oil in a large skillet over medium heat.
Add the diced onion and cook for 3–4 minutes until softened.
Add the Italian sausage and cook for 6–8 minutes, breaking it into small crumbles, until browned and cooked through.
Add the minced garlic and cook for 30–60 seconds until fragrant.
Drain any significant excess grease and allow the sausage mixture to cool slightly.
In a large bowl, combine the ricotta, 1 cup mozzarella, Parmesan, egg, Italian seasoning, parsley, and black pepper.
Fold the cooked sausage and onion mixture into the cheese filling.
Spoon the sausage-and-cheese filling generously into each banana pepper half.
Spoon the remaining marinara around the peppers without completely covering the filling.
Cover the baking dish loosely with foil and bake for 25 minutes.
Remove the foil and sprinkle the remaining 1/2 cup mozzarella over the peppers.
Bake uncovered for another 10–15 minutes, until the peppers are tender and the cheese is melted and bubbling.
Broil for 1–2 minutes if additional browning is desired.
Allow the stuffed peppers to rest for 5 minutes before serving.
Garnish with additional parsley and Parmesan if desired.