Heat oil in large frying pan over medium-high heat. Add onions and garlic, and saute until translucent, about 3 minutes.
Stir in chili powder and cumin powder, and cook for 1 minute.
Add the black beans and water, and cook until beans are tender, about 4 minutes. Use a large fork or potato masher to roughly mash the beans. Cook for another minute.
Preheat oven to 400F. Brush the tostadas lightly with the oil and place on a cookie sheet. Bake for 5 minutes, then turn tostadas, and bake for 3-6 minutes longer until the tostadas are crisp and light brown.
Divide and spread the refried bean mixture onto the 4 tostadas, add toppings, and serve with hot sauce, if desired.
Serves 4 starters/appetizers, 2 mains
Recipe by Vegangela at http://vegangela.com/2013/07/21/refried-bean-tostadas/