Start by rinsing the quinoa under cold water to remove any bitterness. Chop the red and green bell peppers, onion, and garlic, and set them aside.
In your slow cooker, add the rinsed quinoa, vegetable broth, black beans, corn, diced tomatoes (with juices), chopped bell peppers, onion, and garlic. Stir everything to combine.
Add in the chili powder, crushed red pepper flakes, salt, black pepper, cumin, and oregano. Stir the mixture again to ensure that all the spices are evenly distributed.
Set your slow cooker to low and cook for 6-8 hours, or on high for about 3-4 hours, until the quinoa is fluffy and the vegetables are tender.
In the last 30 minutes of cooking, sprinkle the shredded Mexican cheese on top of the quinoa mixture. Cover and allow the cheese to melt into the dish.
Once done, give everything a gentle stir. Serve your Slow Cooker Mexican Quinoa Bake warm, garnished with fresh cilantro, avocado, or a dollop of sour cream if desired.