Cut the tops off of the bell peppers and remove the inside membranes and seeds. Arrange the peppers cut-side up in a rectangular baking dish. 9-inch by 13-inch is a good size.
Cook the quinoa. Rinse it well under cold water, then place in a medium saucepan. Add 1 1/2 cups water and bring to a boil over high heat. Once it comes to a boil, reduce heat to low and cover. Simmer until fluffy and the water is absorbed, 15-20 minutes. Remove from heat and set aside.
Set a large saute pan over medium heat. When hot, add the olive oil. Add the onion, bell pepper, and oregano and cook, stirring occasionally, until the veggies are tender and the onion is a bit browned, 6-7 minutes.
Remove from heat and stir in the cooked quinoa, black beans, diced tomatoes, corn, green chiles, chili powder, cumin, salt, and pepper.
If you're adding chicken to all, stir that in now. If you're only adding chicken to some, first fill the peppers you'd like to be meatless, then stir the chicken into the mix, about 1/4 cup diced chicken per pepper.
Finish dividing the mixture between the bell peppers. Carefully add 1/2 cup of water to the bottom of the baking dish and cover the dish tightly with foil.
Bake until the bell peppers are fork-tender, 30 to 40 minutes. Remove the foil and sprinkle with cheese (omit cheese or use vegan cheese for the dairy-free/vegan versions). Bake until the cheese has browned, about 10 more minutes.
Serve with assorted toppings.
Add cooked, diced chicken to the mix - about a scant 1/4 cup per pepper. I first filled three vegan peppers for myself then mixed in about 3/4 cup cooked chicken for the meaty versions.
Omit cheese or use vegan cheese.
The recipe is written so that you'll have ample filling for just about any size bell pepper, so there's a chance you'll have some left over.