In a large saucepan, heat oil over medium heat. Add chopped onion to the pan, sauté 5 minutes, or until softened, stirring frequently. Add garlic, chili powder and cumin. Cook, stirring until fragrant, about 30 more seconds.
Add tomatoes with their juice, black beans, corn, vegetable stock and water. Stir, and season with coarse salt and freshly ground pepper.
Bring soup to a boil and reduce to simmer.
Add tortilla chips, cook until softened, about 2 minutes. Stir in lime juice, season with salt and pepper.
Divide soup into 4 serving bowls. Top with avocado cubes. Garnish with fresh cilantro and additional crumbled chips, if desired.