Squeeze limes into a medium bowl. Thinly slice scallions, and add to bowl along with corn. Add 1 tablespoon oil, and season with salt and pepper; toss to combine. Cover and refrigerate.
Halve jalapeno (remove ribs and seeds for less heat, if desired), and mince. Halve tomatoes. Combine in an airtight container; cover and refrigerate.
Coarsely grate cheese on a box grater; place into a resealable plastic bag, and refrigerate.
Preheat oven to 475 degrees. Arrange tortillas on two baking sheets; brush both sides with remaining 2 tablespoons oil. Rinse and drain beans; sprinkle over tortillas. Top with tomatoes, jalapeno, and cheese. Bake until golden and crisp, about 10 minutes, rotating sheets halfway through.
While tostadas are baking, pit and peel avocado; dice. Toss with corn mixture to combine.
To serve, top tostadas with corn relish, and, if desired, sour cream.
You can do the first three steps in the morning, and the final three at dinner time.