Heat about half an inch of oil in a heavy skillet or deep pot set over medium-high heat. When it’s hot, but not smoking, fry 4 tortillas—one at a time—until golden and crisp. Transfer to a paper towel-lined plate and season with salt.
To make the ceviche, combine the fish, tomato, cilantro and chili.
Add a generous amount of lime juice and season with salt. Let the mixture sit—the lime juice will “cook” the fish—for 10 minutes. Taste and add more lime juice, if you like.
Serve the ceviche, using a slotted spoon to drain off excess liquid, on the crisp tostadas, topped with salsa, avocado, radishes, cilantro and lime wedges to squeeze overtop.