Preheat oven to 350.
Lightly oil a skillet. Heat on low/med. Add spices to heated skillet and stir for about a minute.
Add beans (with liquid). Simmer beans and mash occasionally with a potato masher.
Place tortillas in heated oven (I put them directly on the oven racks). Heat until crisp (about 10 minutes, but check them).
When beans look like a good spreadable consistency, turn off the heat and cover until ready to use.
Assemble tostadas according to taste. First, spread tortillas with beans. Then add lettuce, tomatoes, onion and salsa.
This is a nice change from burritos and kids seem love to help assemble!