Heat a non-stick skillet over medium heat.
Add diced onions and cook until translucent.
Stir in minced garlic and cook for an additional minute.
Add black beans, corn, diced tomatoes, and taco seasoning; cook for 5-7 minutes.
Prepare the avocado sauce by blending avocado, lime juice, and cilantro.
Warm taco shells in the oven for 5 minutes.
Fill each taco shell with the bean mixture.
Drizzle the avocado sauce on top.
Garnish with fresh cilantro.
Serve immediately.