Add the beans to a large bowl and cover with cold water. Soak overnight. Drain and rinse the beans. Place them in a medium sized pot and cover with water. Cook for 1 1/2 hours. Drain.
Heat a large skillet over medium heat. Add the vegetable oil and the onion, garlic and cumin. Cook for 5 minutes.
Next add the beans and cook for 3 to 4 minutes more. Season with salt and pepper. Using the back of a fork or a potato masher, mash the beans adding water a couple tablespoons at a time while you mash.
For the tostada: Preheat the oven to 400 degrees.
Place the tortillas directly on the oven rack and toast for 5 to 8 minutes. Remove from the oven and allow to cool slightly.
Spread beans on the top of each tortilla and top with lettuce, tomato, sour cream, cheese and hot sauce.