Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes from the inside.
In a medium skillet, heat the olive oil over medium heat and sauté the diced yellow onion and minced garlic until translucent.
Add the ground beef to the skillet, breaking it up with a spoon, and cook until browned.
Stir in the sea salt, black pepper, dried oregano, cumin, and tomato puree, simmering for 2-3 minutes until the flavors meld.
Place the bell peppers upright in a baking dish and fill each pepper generously with the beef mixture.
Add a splash of water to the bottom of the baking dish, cover with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes until the peppers are tender and slightly charred on the edges.