Lay the tomatoes for 1 1/2 hours on ice or in ice water; shortly before serving wipe the tomatoes dry and cut them on a board with a sharp knife into thin slices; also cut the onions into fine slices; put them in alternate layers with the tomatoes into a salad dish and sprinkle over each layer 1/4 teaspoonful salt, 1/4 teaspoonful sugar and half that quantity of white pepper; mix the vinegar with 3/4 cup water, pour it over the whole and serve at once. 2 tablespoonfuls oil may be added to the salad if liked, but many people object to it on tomato salad.