1. In a pan, heat oil over medium-high heat. Add beef and break it down to separate, using a wooden spoon. Add taco seasoning and stir. Let it cook until brown.
2. Add black beans, water and salt. Stir to combine. Lower the heat to medium and let simmer until all the water has cooked down, about 8 minutes. Remove from the heat and set aside.
3. Top one side of a tortilla with beef and beans mixture, cheese and avocado slices. Fold the tortilla to close. Repeat with remaining tortillas to make 4 quesadillas.
4. Please tortillas, 2 at a time, in a large pan over medium heat. Heat the quesadillas, turning occasionally, until the cheese has melted, about 2 minutes.