Heat the canola oil in a skillet over medium-high heat.
Add the chopped onion and red bell pepper and sauté about 3 minutes, until softened.
Add the minced garlic and sauté another 30 seconds, then transfer the mixture to the slow cooker.
To the slow cooker, add the dry quinoa, vegetable broth, canned tomatoes with green chilies (undrained), tomato sauce, chili powder, ground cumin, and salt and pepper to taste; stir to combine.
Cover and cook on HIGH for about 2 hours 45 minutes to 3 hours 15 minutes, checking toward the end so it doesn't dry at the edges or become soggy.
Stir in the frozen corn, drained black beans, and drained pinto beans until evenly distributed.
Smooth the top, sprinkle the shredded cheese evenly over the mixture, then cover and cook until heated through and the cheese melts, about 10–15 minutes more.
Serve warm with desired optional toppings such as diced avocado, diced Roma tomatoes, chopped cilantro, lime wedges, and chopped green onions.
