black-bean tostadass with corn relish

2 limes 2 scallions 1 1/2 c frozen corn 3 tbl olive oil coarse salt and ground pepper 1 jalapeño chile 2 c grape tomatoes 8 oz montery jack cheese 4 flour tortiallas (small) 1 can black beans 1 avocado reduced-fat sour cream for serving
*in the morning start corn relish. squeeze limes into a medium bowl. thinly slice scallions, ad add to bowl along with corn. add 1 tbl oil, and season with salt and pepper; toss to combine. cover and refrigerate. halve jalpeno, remove ribs and seeds and mince. halve tomatoes. combine in airtight container, cover and refrigerate coarsely grate cheese on a box grater, put in a bag, and refrigerate.
*at dinnertime make tostadas. preheat oven to 475. arrange tortillas on two baking sheets, brush both sides with remaining 2 tbl oil. rinse and drain beans, sprinkle over tortillas. top with tomatoes, and cheese. bake ntil 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. top tostadas with corn relish, and if desired, sour cream.