Ceviche is a South American seafood dish where diced raw fish or other shellfish is marinated in citrus. This process is called denaturation - the acid from the citrus essentially “cooks” the fish, turning it from raw to firm and opaque.
Add the fish, onion, tomato, cilantro, lime juice and orange juice to a medium mixing bowl tossing to combine. Season with salt. Cover with plastic wrap and refrigerate until the fish becomes firm and opaque, about 1 hour.
For the spicy crema:
In a small bowl stir together the sour cream, chili powder, cayenne and 2 tablespoons of water until the sauce has thinned and is completely combined. Cover and refrigerate until ready to use.
For serving:
Fill a large cast iron pan or sauté pan with enough vegetable oil to reach ¼-inch up the pan and heat through. Test a corner of one of the corn tortillas, it should bubble when added to the hot oil. Working one at a time, add the tortilla to the hot oil and fry until crispy on both sides, about 1 minute per side. Remove to a paper towel-lined plate and repeat with the remaining tortillas.
Drain the excess liquid from the ceviche. Use a spoon to add a scoop of the ceviche to each of the tostadas and top with slices of avocado, radish, jalapeños and cilantro. Drizzle with the spicy crema and serve immediately.