Heat oil in a large pan over medium/high heat. Add the onion, bell peppers and garlic. Cook for 2-3 minutes or until onion is lightly golden. Add the beans and taco seasoning. Cook for another 2-3 minutes. Remove from heat and cool for a few minutes before assembling.
In a medium bowl, combine the avocado, tomato, cilantro, lime juice and a pinch of salt and pepper.
To assemble, Lay a tortilla on a flat surface. Spread with a teaspoon of sour cream or mayo. Top with a few tablespoons of the black bean mixture and a few tablespoons of the avocado mixture.