Preheat your oven to 375°F (190°C).
Heat olive oil in a medium skillet over medium heat. Add onions and garlic, sautéing until they become fragrant and translucent.
Add the ground beef or sausage to the skillet, cooking until fully browned. Season with oregano, basil, salt, and pepper.
Remove the skillet from the heat and allow the mixture to cool slightly. Stir in mozzarella and parmesan cheese until well combined.
Stuff each banana pepper with the meat and cheese mixture, ensuring they are fully filled but not overstuffed.
Place the stuffed peppers in a baking dish and pour marinara sauce over the top.
Cover the dish with foil and bake for 30 minutes. Then, remove the foil and bake for an additional 10-15 minutes until peppers are tender and the cheese is bubbly and golden brown.
Let the peppers cool slightly before serving.
