Rinse quinoa under cold running water for a moment or two. Place in a medium sauce pan with vegetable stock and bring to a boil. Cover and reduce heat to a simmer for 15 minutes. Remove from heat while leaving lid on and let sit for 10 minutes, then fluff with a fork and allow to slightly cool.
Preheat oven to 400° F.
In a large bowl mix together cooked quinoa, beans, tomatoes, cheese, chili powder, cumin, garlic powder, onion powder, and chopped cilantro (if using). Spoon mixture into halved peppers and bake for 20-25 minutes or until peppers have started to soften and cook through.
Cotija is a hard-crumbly Mexican cheese that can be replaced with feta.