Preheat oven to 375 degrees Fahrenheit.
In a large skillet over medium-high heat, cook the ground turkey until fully browned and no pink remains.
Stir in the cooked quinoa, black beans, corn, cumin, chili powder, and garlic powder into the skillet.
Pour 1 cup of marinara sauce into the turkey mixture and stir to combine.
Arrange the bell peppers upright in a baking dish and fill each with the turkey mixture.
Pour the remaining 0.5 cup of marinara sauce over the tops of the peppers and cover the dish tightly with foil.
Bake for 30 minutes, then remove the foil and sprinkle each pepper with shredded cheese.
Bake uncovered for an additional 10 minutes or until the peppers are tender and the cheese is melted.