In a medium skillet over medium heat, warm oil. When hot, add onion; cook for 5-7 minutes, until lightly browned. Add garlic; cook for 1 minute, stirring. Add tomatoes, corn, and bell pepper; cook for 5-7 minutes, until vegetables are tender.
Add sweet potato and 1/2 cup water; stir. Cover skillet; cook for 15 minutes, until sweet potato is fork tender. Add black beans, salsa, chili powder, cumin, and pepper; stir and cook for 5 minutes.
Fill each tortilla with 1/2 of the filling. When ready to eat, top with avocado slices.