To toast the tortillas in the oven: Preheat the oven to 375 degrees F. Spread the tortillas on a baking sheet. Bake for 10 minutes, or until crisp and dry and just starting to be touched with golden brown spots. Remove them from the oven and place on a serving platter.
To toast the tortillas on a stovetop: Heat a large skillet. Toast the tortillas over medium heat (two or three at a time, depending on the size of the skillet) for about 5 minutes or so on each side, until crisp and touched with golden brown spots. Don't be afraid to let them get nice and crisp—that's better than ending up with a soggy tostada.
Heat the oil in medium skillet. Add the onion and garlic and sauté until golden, about 5 minutes.
Add the remaining ingredients (aside from the garnishes, of course) along with ¼ cup water and bring to a simmer. Using a potato masher, mash some of the beans so that the liquid becomes thick and saucy.
Place the shredded lettuce, sour cream, and salsa in separate serving bowls and let everyone assemble their tostadas as follows: A layer of shredded lettuce; the black bean mixture; salsa; and sour cream. Pick up the tostadas and eat out of hand (with plenty of napkins!).