Use very fresh, sushi-grade fish or seafood
Cut the seafood into small, even pieces
Place the seafood in a non-reactive bowl
Add fresh lime or lemon juice until fully covered
Mix in salt
Refrigerate and let it marinate until opaque and firm
Drain off excess citrus juice if needed
Add diced onion
Add chopped cilantro
Add diced tomato, cucumber, or chili if desired
Toss gently to combine
Taste and adjust salt and citrus
Serve chilled immediately