Preheat the oven to 375°F (190°C).
Cut the tops off the bell peppers and remove the seeds.
In a skillet, heat olive oil over medium heat.
Add diced tomatoes and cook for 5 minutes.
Stir in the black beans, cooked quinoa, cumin, and chili powder.
Season with salt and pepper to taste.
Fill each bell pepper with the mixture.
Place stuffed peppers in a baking dish.
Cover with foil and bake for 25 minutes.
Remove the foil and bake for an additional 5 minutes.