Black Bean-And-Rice Tostadas

cooking spray

1 cup chopped onion

2 garlic cloves -- minced

1 cup hot cooked long-grain rice

15 ounces no-salt-added black beans -- rinsed and drained

14 1/2 ounces mexican style stewed tomatoes -- undrained

--with jalapeno peppers and spices

7 ounces corn with red and green peppers -- drained

4 1/2 ounces chopped green chiles -- drained

2 tablespoons lime juice

1/2 teaspoon dried oregano

1/2 teaspoon ground cumin

1/2 teaspoon chili powder

1/4 teaspoon ground red pepper

4 fat-free (6-inch) flour tortillas

1/2 cup finely shredded reduced-fat sharp cheddar

-- cheese

1/4 cup fat-free sour cream