Preheat oven to 350 degrees F. In a large saucepan, bring water to boiling. Stir in rice and onion. Return to boiling; reduce heat. Cover and simmer for 5 minutes. Remove from heat. Stir. Cover and let stand for 5 minutes. Stir undrained chili beans and the corn into rice mixture. Heat through.
Meanwhile, place tostada shells on a baking sheet. Bake for 5 minutes or until heated through.
To assemble, place 2 tostada shells on each dinner plate. Top tostadas with shredded lettuce and the rice-bean mixture. Sprinkle with cheddar cheese and top with tomatoes. Makes 4 servings.