1. Reconstitute the corn husks.

2. In a small saucepan, combine the black beans, avocado leaves, lard, onion, and 3 cups water. Set saucepan over high heat. Bring the liquid to a boil. Reduce the heat to medium-low. With lid slightly ajar, simmer until the beans become very tender, about 1 1/2 hours. Add additional water as necessary if the beans begin to dry out during the cooking process.

3. Remove and discard the avocado leaves from filling. Generously season filling with salt. Remove pan from heat, and set the filling aside to cool.

4. Using a potato masher or a large spoon, coarsely mash the filling until it becomes thick enough to easily hold its shape when placed in a spoon.

5. Assemble 16 tamales with the tamale batter and the black-bean filling, using large husks for wrappers and ties, and reserving smaller ones to line the steamer basket and cover the tamales.

6. Prepare the steamer, and steam the tamales. Let the tamales stand until the batter has firmed. Serve the tamales warm with the chopped-tomato-and-serrano salsa.