Directions

1. Combine black beans, water, 1 tablespoon lime juice and taco seasoning in a microwave-safe bowl. Microwave on High for 2 minutes. Using a fork, mash the beans into a paste; set aside.
2. Mash avocado with a fork in a small bowl. Add tomato, onion, the remaining 1 tablespoon lime juice and a pinch of salt. Set aside.
3. Coat tortilla on both sides with cooking spray. Heat a small nonstick skillet over medium heat. Add the tortilla to the pan and toast, flipping once, until crisp and lightly browned on both sides, 3 to 4 minutes. Transfer to a plate.
4. Crack egg into the pan and reduce heat to medium-low. Cook, flipping once if desired, until the white is set, 2 to 3 minutes.
5. To assemble the tostada, layer the bean mixture, avocado mixture and lettuce on the tortilla. Top with the egg.