Cut the tops off the bell peppers and remove the seeds and membranes. Place the peppers in a baking dish.
In a large skillet, cook the ground beef over medium heat until browned. Add the diced onion and garlic, and cook until softened.
Stir in the cooked quinoa, diced tomatoes (with juices), oregano, cumin, paprika, salt, and pepper. Cook for an additional 5 minutes.
Spoon the beef and quinoa mixture into each bell pepper until filled to the top.
Cover the baking dish with foil and bake in the preheated oven for 25-30 minutes, or until the peppers are tender.
Remove the foil, sprinkle the shredded cheddar cheese on top of each stuffed pepper, and bake for an additional 5 minutes until the cheese is melted and bubbly.