For serving: chopped fresh cilantro, diced avocado, chopped green onion, sour cream or plain Greek yogurt
Heat the olive oil in a large skillet or Dutch oven over medium high. Add the turkey and onion. Cook and stir, breaking up the turkey as you go, until the turkey is no longer pink, about 5 minutes. Transfer to the bottom of a large slow cooker.
To the slow cooker, add the quinoa, enchilada sauce, black beans, tomatoes, corn, red bell pepper, green bell pepper, chili powder, cumin, garlic powder, and 1/2 cup water. Stir to combine, then cover and cook on high for 2 1/2 to 3 hours or low for 5 to 6 hours, until the liquid is absorbed and the quinoa is tender.
Remove the lid and stir. Taste and adjust any seasonings as desired. Stir in 1/2 cup of the shredded cheese, then sprinkle the remaining cheese over the top. Cover and cook on high until the cheese melts, about 10 to 15 minutes. Serve hot with any desired toppings.