Heat the oil in a large nonstick skillet to medium. Add garlic and onion and sauté until onion is just tender, about 3–4 minutes.
Add the black beans and red beans and sauté for 3–4 minutes.
Mix in the salt, paprika, cayenne pepper, cumin, and corn kernels.
In a small bowl, whisk together the marinara sauce and green chiles. Pour the marinara–green chile mixture into the skillet with the bean mixture and combine. Stir in the cilantro.
Stuff the peppers with the bean mixture. Arrange the peppers in a 5- or 6-quart slow cooker. Cover and cook the peppers on high for 2 hours or on low for 4 hours. Remove the lid and top the peppers with the shredded cheese, replace the cover, and cook on high for 15 to 20 additional minutes. Allow peppers to cool 10–15 minutes before serving. Serve warm.
