1. Wash hands with soap and warm water. Wash all produce well and chop as directed.
3. Heat olive oil in frying pan. Add the onion and sauté over medium-high heat for about 5 minutes.
4. Add the beans, water, chili powder, garlic, oregano, cumin and salt. Cook over medium heat 5 to 10, stirring occasionally.
5. Remove from heat and use a potato masher or fork to mash beans.
6. Lightly spray the tortillas with oil on both sides. Place them on a baking sheet. Bake for about 5 minutes until golden and crispy. Remove from the oven and place on a serving dish.
7. Spread about 3 tablespoons of the mashed beans over each tostada. Sprinkle lettuce, tomatoes, radishes and cheese on top of each tostada.