Very finely chop the onion. Beat the egg slightly.
In a mixing bowl, using your hand, combine all the ingredients well, then season with salt & pepper to your taste.
Shape 4-6 balls of the mixture and flatten them slightly between your palms. If you have time, cover and rest in the fridge for half an hour.
Brush the patties lightly with oil, then griddle, grill, fry or barbecue for about 5 minutes on each side.
Meanwhile, cut the rolls in half, spread mayonnaise on the bottom half, cover with lettuce leaves, then lay tomato and gherkin slices. Place a patty on it, then add a couple of drops of Tabasco, and top with the other half of the bun.