1. Preheat oven to 350°F. Bring 1/2 cup salted water to a boil in a small saucepan. Stir in quinoa, cover, reduce heat to low and simmer until tender, about 15 minutes. Remove from heat. Place peppers upright in an 8-inch square baking dish. Bake 15 minutes.
2. In a large skillet, warm oil over medium heat. Sauté onion until soft, about 7 minutes. Add garlic; sauté 1 minute. Add turkey; cook, stirring, until no pink remains, about 7 minutes. Stir in tomatoes, beans, corn and quinoa. Raise heat to medium-high. Bring to a boil. Reduce to medium-low. Simmer, stirring, until thick, about 5 minutes. Remove from heat. Stir in feta and season with salt and pepper.
3. Stuff peppers with filling, mounding slightly. Bake until peppers are tender, 15 to 20 minutes. Let stand 5 minutes; serve.