Preheat oven to 400 °F.
Arrange the peppers, cut-side up, in a 9x13-inch baking dish.
Add two tablespoons extra virgin olive oil into the pan. Combine the sausage and onion in a large skillet over medium heat. Cook, breaking up the meat with a wooden spoon, until the meat is browned. Stir in the garlic, oregano, basil, and a pinch each of salt and pepper. Cook, until fragrant, 1 minute more. Remove the skillet from the heat.
Divide the meat filling among the peppers, then slather the marinara sauce all over the peppers. Top with the mozzarella. Bake until the cheese is melted and getting bubbly and brown 20-25 minutes.
Garnish the peppers with fresh basil and serve.