Step 1Salt and airdry the fish: Generously salt both sides of the fish fillets, about 1/2 teaspoon per ½-pound fillet. Place the fish on a rack set over a sheet pan and refrigerate, uncovered, to “sweat,” 10 to 15 minutes. Rinse the fish in cold water, and dry well. Refrigerate, on a rack, uncovered for at least 3 hours, up to overnight. (If holding more than 8 hours, cover the fish with cheesecloth).
Step 2Cut the fish: Cut the fillet strips lengthwise along the natural divisions so they are no more than 1 inch wide or high. Trim away any dark bits and ends, and save these for the Leche de Tigre (you will need 2 tablespoons). Cut each strip crosswise into ¼- to ½-inch thick pieces. Refrigerate until ready to make the ceviche (this step may be done 1 to 4 hours ahead).
Step 1Make the leche de tigre no more than three hours before preparing the ceviche. In a blender, combine the fish, garlic, ginger, celery, onion and ¼ cup of the lime juice and puree until the ingredients are well combined. With the blender running, add the salt, then very slowly pour in the remaining lime juice so the sauce emulsifies. With the blender still running, add the ice cubes a few at a time until the sauce is smooth and frothy. Use immediately, or refrigerate the leche de tigre for up to 3 hours.
Step 1Deflame the onion: Cut the onion in half lengthwise. Cut one half into ¼-inch dice and place in bowl. Cut the remaining half crosswise into the thinnest possible slices and place in a separate bowl. Add 1 ½ teaspoons salt and 1 cup hot water to each bowl. Stir and set aside for 10 minutes. Rinse, pat dry and return the onions to their bowls.
Step 2Make and serve the ceviche: Just before serving, place the fish cubes, diced onion, chiles to taste and cilantro in a large non-reactive bowl. Add a handful of ice cubes to the leche de tigre and stir briefly to thicken the mixture and make it creamy. Discard the ice cubes. Pour leche de tigre over fish and toss together. This makes about 1 quart ceviche. Transfer the mixture to a shallow serving bowl, mounding the fish in the center. Place the grilled sweet potatoes, corn and cubes of avocado around the fish (or artfully fan avocado slices). Top the fish with a generous tangle of sliced onions. Alternatively, divide the ceviche, avocado, sweet potatoes, corn and sliced onions among individual shallow bowls. Serve cold and pass the remaining accompaniments separately.