1. Preheat oven to 350ºF. Arrange tortillas on a baking sheet, mist with cooking spray and sprinkle with salt. Bake until beginning to crisp, 5 to 7 minutes.
2. While tortillas are toasting, combine beans, corn, avocado, cilantro, olive oil, lime juice and red pepper flakes in a large bowl. Top tortillas, still on baking sheet, with some bean and corn mixture. Sprinkle with Cheddar.
3. Turn oven to broil and broil tostadas until cheese has melted and is beginning to brown, 1 to 2 minutes. Serve immediately.