In a large glass or stainless steel bowl, combine fish, chopped onions and lime juice. If the lime juice doesn’t cover the fish, you may need to squeeze in a little more. Cover the bowl and refrigerate until fish reaches the desired doneness (about 1 hour for medium-rare). Pour mixture through a medium-mesh strainer to remove all of the lime juice. Transfer cooked fish and onions back to the bowl and discard lime juice.
To the fish mixture, add jalapeno, tomato, 1/4 cup cilantro and olive oil, stirring to combine. Add sugar, then season to taste with kosher salt (I added just under 1 teaspoon). Refrigerate until ready to use (no longer than 2 hours).
Combine cilantro leaves, lime juice and mayonnaise in a food process and puree until smooth. Season to taste with kosher salt and black pepper.
Spread a thin layer of cilantro cream on each tostada. Top with ceviche, a slice or two of avocado and a scattering of chopped cilantro.