Preheat your oven to 375°F (190°C). Slice the tops off the bell peppers and remove the seeds and internal membranes.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Add the minced garlic and ground bison to the skillet. Brown the meat, breaking it into small crumbles with a spatula, until fully cooked through.
Stir in the cooked quinoa, smoked paprika, cumin, sea salt, and black pepper. Mix until the spices are fragrant and the mixture is well combined.
Stuff each bell pepper tightly with the bison and quinoa mixture, pressing down slightly to pack it in.
Place the peppers upright in a baking dish. Spoon one tablespoon of tomato sauce over the top of each stuffed pepper.
Pour 1/4 cup of water into the bottom of the dish to create steam, cover the dish tightly with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes until the peppers are tender and the tops are slightly caramelized.