Step 1Deflame the onion: In a medium bowl, combine the chopped onion with 2 teaspoons salt. Stir in 1 cup hot water, and set aside for 10 minutes. Drain the onions and reserve the soaking water, pressing on the onions to extract all liquid. Rinse the onions and pat dry (this step may be done several hours ahead). Place the onions in large mixing bowl.
Step 2Cut the florets vertically into ¼-inch-thick slices. Cook 2 minutes in boiling salted water, drain, then chill with ice cubes to stop the cooking process. Drain again and pat dry (this step may be done up to 1 day ahead). Add the cauliflower to the onions.
Step 3Shave the corn from the cob and add to bowl. Seed the tomatoes and discard the pulpy bits. Cut the tomato “meat” into small dice and add to the bowl. Add the beans, mushrooms and hearts of palm. Dish can be prepared to this point several hours ahead and refrigerated.
Step 4A few minutes before serving, stir in the cilantro, 1/3 cup of the lime juice and reserved onion water. Taste the mixture; it should be tangy and flavorful. Add salt and remaining lime juice as needed. If it’s too acidic for your taste, stir in a few ice cubes for a moment or two to dilute, then discard the ice. Gently stir in the avocado. This makes about 2 quarts ceviche.
Step 5To serve, place the ceviche in a large shallow bowl, or divide among individual bowls, and pass the garnishes separately.