Seviche

Add sliced onions to a saucepan over medium heat and add just enough water to cover. Bring to a simmer and cook just enough to soften. Remove from heat and drain. Rinse with cold water and drain.

Score tomato with an X, skin deep, on the top and bottom. Place in the same saucepan used for the onions and add enough water to cover. Bring to a simmer and cook until soft and the skin starts to peel, about 5 minutes. Remove tomato and rinse under cold water until cool enough to hold in your hand. Using your fingers or the blade of a knife, gently peel off the skin. Place peeled tomato in a blender and purée.

Combine onions, lemon juice, and olive oil in a large glass dish or bowl. Stir in oysters, puréed tomato, diced tomato, green onions, ketchup, and soy sauce. Season with parsley, garlic salt, sugar, and salt to taste.

Marinate in the refrigerator for at least 30 minutes if using canned oysters. Increase marinating time to at least 2 hours in the refrigerator if using raw shrimp, scallops, or fish, until they are opaque in the middle.