Cut bell peppers in half, remove and discard stems, membranes, and seeds. Spray the inside of your slow cooker with nonstick spray. Place peppers in the slow cooker side by side (squish them a bit if you need to). ***see note***
In a bowl, stir together uncooked quinoa, salsa, green chiles, black beans, corn, and seasonings. Spoon mixture into the peppers inside the slow cooker. Cover and cook on high for 3 hours.
About 30 minutes before serving, uncover slow cooker. Sprinkle cheese on top of stuffed peppers. Cover and allow to cook another 20-30 minutes (the cheese will get melty). Serve warm and top with cilantro, sour cream, and jalapeños as desired.