For tostada shells, place tortillas in a single layer directly on the middle oven rack. Bake in a 350 degree F oven about 6 minutes, turning halfway through baking time, or until golden and crisp. (If tortillas bubble during baking, puncture the bubble with a fork.) Set aside and cover to keep warm.
Meanwhile, in a 10-inch skillet cook and stir chicken and chili powder over medium heat for 5 to 7 minutes or until chicken is cooked through. Stir in salsa. Set aside; keep warm.
Drain beans, reserving liquid. In a small saucepan stir beans over low heat until heated through. With a potato masher or fork, mash beans adding enough reserved bean liquid to make spreadable consistency. Heat through.
To assemble, place a warm tortilla on each dinner plate. Spread with a thin layer of beans. Top beans with some of the chicken mixture, pimiento, lettuce, radishes, cheese, olives, sour cream (if desired), and green onion. Garnish with avocado, if desired. Makes 4 servings.