Heat the olive oil in a large pot or Dutch oven over medium heat. Add the onion, bell peppers, and carrots, and cook for 6-8 minutes until softened.
Stir in the garlic and cook for 1 minute until fragrant.
Add the mushrooms and walnuts and cook for 5-6 minutes, stirring occasionally, until the mushrooms release their liquid and start to brown.
Stir in the chili powder, cumin, and smoked paprika, and cook for 1 minute to toast the spices.
Add the crushed tomatoes, kidney beans, black beans, corn, and vegetable broth. Stir well and bring to a simmer.
Reduce the heat to low and simmer uncovered for 25-30 minutes, stirring occasionally, until thickened. Season with salt and pepper to taste.
Ladle into bowls and top with shredded cheddar, a dollop of sour cream, and sliced green onions. Serve with cornbread on the side.
