Slice 1 medium firm tomato about 1/16 inch thick with a serrated knife; pat dry with paper towels (discard the end pieces). Sprinkle with salt and let sit 15 minutes, then blot the excess moisture with paper towels. Arrange in a single layer on a flat microwave-safe plate coated with cooking spray. Mist the slices with cooking spray, then microwave until they start drying out, about 6 minutes. Carefully flip; microwave until stiff and mostly dry, 30 seconds to 1 minute. Transfer the chips to a rack to cool.