Preheat oven to 400°F. Mist every other cup of a 12-cup muffin tin with nonstick cooking spray. Press a tortilla into each sprayed cup, flattening out edges, and bake until golden, about 8 minutes.
Divide beans among tostadas, top with salsa and sprinkle with cheese. Return to oven and bake until warmed through, 7 to 10 minutes.
Place each tostada on a plate and sprinkle with cilantro. Top with some lettuce and a small dollop of sour cream. Serve with extra salsa, if desired.