Add the lentils, onion, bell peppers, garlic, black beans, kidney beans, diced tomatoes, tomato sauce, vegetable broth, chili powder, cumin, smoked paprika, oregano, salt, black pepper, and red pepper flakes, if using, to the Crock-Pot.

Stir everything well to combine.

Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours, until the lentils are tender and the chili has thickened.

During the last 20 to 30 minutes of cooking, stir in the frozen corn.

Turn off the slow cooker and stir in the lime juice.

Taste and adjust seasoning with additional salt if needed.

Ladle into bowls and top with fresh cilantro and any desired toppings such as cheese, sour cream, avocado, or tortilla chips.