Place the fish and carrots in a food processor and pulse 5-6 times, or until the mixture is finely chopped, taking care not to turn it into a puree. Alternatively you could mince with a sharp knife.
Place the fish mixture in a large bowl, pour the lime juice over it, and gently toss. Cover and marinate at room temperature for 30-45 minutes.
Drain the fish mixture through a fine mesh sieve, pressing out as much liquid as possible with the back of a spoon. Return the fish to the bowl and stir in the chile, onion, cilantro, oregano, salt, and pepper. Mix well. Cover and refrigerate for at least 30 minutes, or up to 12 hours, so all the flavors come together.
When ready to serve, taste the ceviche for salt and pepper and add more if necessary.
Mound 2-3 tablespoons of ceviche on top of each tostada. Top each with a slice of tomato and avocado and a final sprinkling of salt.
Credit Pati Jinich