In a heatproof bowl, soak chili pepper in boiling water for 30 minutes. Drain and discard stems and water. Set aside.
In a skillet, heat olive oil over medium heat. Add onions and cook till soft, about 5 minutes. Add garlic, cumin, chili powder, lime zest, salt, pepper, and reserved chili and cook for an additional minute. Transfer mixture to the food processor along with beans and 1 cup tomato liquid and process till smooth.
Transfer processed mixture to a slow cooker. Add the remaining tomatoes and broth, mixing well. Cover and cook on low for 8 to 10 hours or high for 3 to 4 hours. Stir in green mild chilies and corn. Cover and cook on high for another 15 to 20 minutes.
Meanwhile, preheat oven to 400 F degrees. Brush tortilla strips with oil, place on the cookie sheet and bake for 4 minutes on each side, till crispy.
To serve, ladle soup into bowls and top with tortilla strips, avocado, red onion, sour cream, and cilantro.