Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers, remove the seeds and membranes, and set the peppers upright in a small baking dish.
In a large skillet over medium heat, add the avocado oil and sauté the ground beef and diced yellow onion until the beef is browned and the onion is translucent.
Add the minced garlic, dried oregano, sea salt, and black pepper to the skillet, cooking for 1 minute until highly fragrant.
Remove the skillet from the heat and stir in the cooked brown rice and 2 tablespoons of the tomato puree until well combined.
Spoon the beef and rice mixture into the hollowed bell peppers, pressing down gently to pack them tightly.
Top each pepper with the remaining tomato puree and a sprinkle of parmesan cheese.
Pour the water into the bottom of the baking dish to create steam, then cover the dish tightly with foil.
Bake for 30 minutes, then remove the foil and bake for an additional 10-15 minutes until the peppers are tender and the cheese is golden and bubbly.