Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until softened, about 5 minutes.
Stir in the minced garlic, chili powder, cumin, and cayenne pepper. Cook for 1 minute until the spices smell fragrant.
Pour in the vegetable broth, then add the black beans, corn, and diced tomatoes. Season with salt and black pepper.
Bring the soup to a simmer and cook for 15 minutes to let the flavors blend.
While the soup simmers, spread the tortilla strips on a baking sheet and bake at 400°F for 8 to 10 minutes until crisp.
Ladle the soup into bowls and top with the baked tortilla strips, cilantro, lime wedges, and avocado slices.