Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and internal membranes.
Heat the olive oil in a large skillet over medium heat.
Add the diced yellow onion and sauté for 3-4 minutes until it becomes translucent.
Add the ground turkey and minced garlic to the skillet, breaking the meat apart with a spatula until it is browned and fully cooked.
Stir in the cooked quinoa, half of the tomato sauce, dried oregano, sea salt, and black pepper.
Pack the turkey and quinoa mixture tightly into each of the prepared bell peppers.
Place the stuffed peppers upright in a baking dish and spoon the remaining tomato sauce over the top of each one.
Pour 0.25 cup of water into the bottom of the baking dish and cover the dish tightly with foil.
Bake for 30 minutes, then remove the foil and continue baking for another 10-15 minutes until the peppers are tender and the tops are slightly golden.