Preheat your oven to 375°F and prepare a baking dish with a thin layer of water to help steam the peppers.
Slice the bell peppers in half lengthwise and remove the seeds and ribs to create a hollow cavity.
Heat the olive oil in a large skillet over medium-high heat and sauté the onion, garlic, and zucchini until softened.
Add the ground beef to the skillet and cook until browned, using a spatula to break the meat into small pieces.
Stir in the tomato sauce, dried oregano, sea salt, and black pepper, allowing the mixture to simmer for 3 minutes.
Spoon the beef and vegetable filling into each bell pepper half, packing it down gently.
Arrange the stuffed peppers in the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil, sprinkle each pepper with parmesan cheese, and bake for an additional 5 minutes until the cheese is melted.