Combine 1 cup diced onion, 1 cup diced bell pepper, cilantro, oregano (or Italian seasoning), garlic, lemon juice, and ancho chile peppers in a food processor,
Transfer mixture to a 6-quart electric slow cooker. Stir in remaining onion, 1 cup bell pepper, vegetable stock, beans, and tomato puree; cover and cook on LOW 7.5 hours.