Put oil in a skillet. Add the onions and garlic and sauté for a couple of minutes. Add black beans and season with salt and pepper. Cook for 3 more minutes.
Cut avocado in half and scoop out insides with a spoon. Put in a mixing bowl and add lime juice. Add tomato, red onion and cilantro and some salt and pepper to taste. Mix together by mashing with a potato masher or fork
Place one tortilla on a plate or cutting board, spread the beans on the tortilla, then the guacamole. Sprinkle with grated cheese and place second tortilla on top.
Slide tortilla into the warm skillet, press tortilla with a spatula and cook over medium heat for 2-3 minutes until bottom tortilla is crispy and cheese is melted.
Flip tortilla and cook for 2 more minutes.
Place on cutting board or plate, cut into 4 triangles and serve hot.