Prep: 1. Preheat oven to 475. 2. Defrost corn by running under hot water. 3. Dice avocado and squeeze limes over them, in a medium bowl. 4. Thinly slice scallions. 5. Mince jalapeño (remove ribs and seeds for less heat, if desired) and halve tomatoes. Set aside. 6. Coarsely grate cheese. Make: 1. Arrange tortillas on two baking sheets; brush both sides with 1 tablespoon oil. 2. Rinse and drain beans; sprinkle over tortillas. 3. Top with tomatoes, jalapeno, and cheese. 4. Bake until golden and crisp, about 10 minutes, rotating sheets halfway through. 5. While tostadas are baking, mix corn, avocado, lime juice, scallions and toss with 1 tablespoon oil, salt and pepper to taste. 6. To serve, top tostadas with corn mixture, and, if desired, sour cream.