Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and internal membranes.
Finely dice the yellow onion and mince the garlic clove.
Heat the olive oil in a large skillet over medium heat and sauté the onion until it becomes translucent.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until it is fully browned.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for an additional minute until fragrant.
Incorporate the cooked brown rice and tomato puree into the beef mixture, simmering for 2-3 minutes to meld the flavors.
Place the bell peppers upright in a baking dish and spoon the beef and rice mixture into each pepper cavity.
Pour a small amount of water into the bottom of the dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil and bake for another 10 minutes until the peppers are tender and the tops are slightly golden.