– 1 pound longaniza, , raw – 1 red onion (thinly-sliced), large (or two small) – 2 tablespoon olive oil – 1 teaspoon vinegar , I used fruit vinegar – ½ teaspoon salt – 4 tablespoon mayonnaise – 4 tablespoon ketchup – 4 panes de agua, (or mini-baguettes) – 4 tablespoon butter (salted) – 2 tomato, very ripe, sliced thinly – 8 leaves of lettuce, use curly or leaf lettuce