Preheat the oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes from the inside.
Heat the olive oil in a large skillet over medium heat and sauté the diced yellow onion until softened and translucent.
Add the ground beef to the skillet, breaking it up with a wooden spoon until fully browned and cooked through.
Stir in the minced garlic, sea salt, black pepper, cumin, and chili powder, cooking for 1 minute until fragrant.
Fold in the diced tomatoes and cooked quinoa, allowing the mixture to simmer for 3 minutes to meld the flavors.
Spoon the beef and quinoa filling generously into the hollowed bell peppers.
Place the peppers upright in a baking dish and add 2 tablespoons of water to the bottom of the dish to create steam.
Cover the dish tightly with foil and bake for 30 minutes, then remove the foil and bake for an additional 10 minutes until the peppers are tender.
Garnish with fresh chopped parsley and serve immediately.