Make the filling: Heat a small heavy skillet or a comal over medium heat until hot. Add garlic, and cook until soft and well charred on all sides, 5 to 10 minutes. Peel, and transfer to the jar of a large heavy-duty blender. Add chile to pan, and toast for 1 minute per side. Tear chile into pieces, discarding stem; add to blender. Add avocado leaves to pan, a few at a time, and toast about 30 seconds per side; tear into pieces, discarding tough stems; add to blender.
In a large deep skillet, heat lard over medium heat. Add onion, and cook until golden brown, about 3 minutes. Using a slotted spoon, transfer onions to blender along with 1/2 cup water. Blend until smooth. Add beans, with their liquid, and blend until smooth. If necessary, work in batches.
Reheat lard in skillet, adding a little more if necessary. Add the blended bean mixture, and cook over medium heat, stirring frequently, until you have a thick paste, about 10 minutes. Season with salt. Let stand at room temperature, covered, or set over an ice bath, until cool.
Fill tamale steamer or a stockpot lined with a steamer with enough water to come just below the level of the rack. Place four light coins in the water. Discontinuation of their jiggling will let you know that the water is getting dangerously low and should be supplemented with more boiling water. Place a double layer of corn husks to cover the rack; set aside.
Make the dough: In a medium bowl, work lard and salt into the masa until evenly distributed. Divide dough into 15 portions, and roll each into a smooth ball.
Line a tortilla press with a piece of matte plastic wrap (such as a cut-open vegetable bag from the grocery store, not a shiny piece of plastic). Place one of the balls on the bottom plate of a tortilla press, and flatted slightly with your hand. Press the top plate down, but not too hard, to obtain a disk of dough about 5 1/2 inches in diameter.
Remove disk from tortilla press. Peel back the top piece of plastic to release the dough, replace the plastic, and flip disk and plastic over. Peel back the second side. Spread a generous tablespoon of the bean paste over the center of the dough, leaving a 1/2-inch border. Use the bottom piece of plastic to fold the dough up and over the filling about two-thirds of the way. Repeat on second side, enclosing the filling. Fold in the two remaining sides, creating a rectangular tamale, about 3 1/2-by-2-inches.
Carefully transfer tamale to a soaked corn husk. Top with a piece of hoja santa. Fold the corn husk over to completely cover dough. Set tamale on a tray, and repeat with remaining ingredients.
By this time the water in the steamer should be boiling and the coins rattling. Place the tamales in the steamer, overlapping them slightly, but not stacking them. Cover with more husks, a thick kitchen towel, and a tightly fitting lid.
Cook tamales for 40 minutes to 1 hour. To test for doneness, remove one of the tamales and check that, when it's opened up, the dough separates easily from the husk. Serve with salsa de chile pasilla de Oaxaca.