1. Marinate the onion : cut the onion into half rings, put it in a bowl, add salt, sugar, vinegar and pour boiling water. Leave for 15 minutes.
2. Cut the sweet pepper into strips, cut the tomatoes into halves. Drain the liquid from the sea cocktail.
3. In a bowl, put the sea cocktail, pepper, tomatoes, pickled onions, a little salt and season with mayonnaise.