In a medium skillet heat the oil over medium heat. Add the onions, peppers and garlic. Season with salt and pepper. Cook until softened, stirring often, about 10 minutes. Meanwhile to your crock-pot add chopped chicken, the Ro-tel, chicken broth, corn, black beans (if using), chile powder, cumin, oregano and 1 teaspoon coarse salt or 1/2 teaspoon fine and about 1/4 teaspoon black pepper. Mix in the cooked onions, peppers and garlic. Cook on high 3 - 4 hour or low 5 - 6. Stir the cheese into the soup. Add the sour cream to a medium bowl and temper it so it doesn't curdle by mixing in a few ladles of the soup broth then mixing it into the soup. . Taste and season as needed. Cook 1/2 to 1 more hour on low heat. Serve garnished with more cheese, sour cream, tortilla chips and avocado.