Preheat oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes.
In a skillet over medium heat, add the olive oil and yellow onion, sautéing until translucent.
Add the ground beef to the skillet, breaking it apart and cooking until browned.
Stir in the cooked brown rice, tomato puree, sea salt, black pepper, garlic powder, and dried oregano.
Simmer the mixture for 3 minutes until the flavors meld and the sauce thickens.
Place the hollowed bell peppers in a baking dish and stuff them tightly with the beef and rice mixture.
Sprinkle the tops with parmesan cheese.
Pour the water into the bottom of the baking dish and cover with foil.
Bake for 30 minutes, then remove the foil and bake for an additional 10 minutes until the peppers are soft and the cheese is golden.