Heat the olive oil in a medium skillet over medium-high heat. Add the beans and the liquid. Cook over medium heat. While the beans are boiling mash them with a potato masher. Continue to cook until they form a thick paste, about 10 minutes. Season to taste with salt.
In heavy skillet, over high heat, heat one-quarter inch of oil. Lower heat to medium and fry tortillas, one at a time, in hot oil for 20 to 40 seconds on each side or until crisp and golden. Drain on paper towels. Store bought tostadas can also be used.
Top 4 tostadas with guacamole and 4 with refried beans. Top each with lettuce, tomato, and cheese. If you want a heartier tostada add bite-size pieces of cooked chicken, pork, or beef. Drizzle with your favorite homemade salsa.
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