Heat the oil in a large skillet over medium-high. Add the onions and bell pepper; cook, stirring often, until just tender, 4 to 5 minutes.
Add the garlic, cumin, and chile powder; cook, stirring often, 1 minute.
Stir together the onion mixture, black beans, tomatoes, water, corn, quinoa, and salt in a 6-quart slow cooker.
Cover and cook on LOW until the quinoa is tender and the liquid is almost absorbed, 4 to 5 hours.
Add the cilantro to the slow cooker, stirring gently to combine.
Sprinkle the cheese over the quinoa mixture; cover and let stand until the cheese is melted, about 10 minutes.
Divide the mixture among 6 bowls; top evenly with the avocado.
