1 Peel and finely chop the onion. Sweat in 1 tablespoon of oil. Cool slightly, then mix with the mustard, egg, and the meat. Mix in the breadcrumbs until the mixture is easy to shape. Season to taste with salt and pepper, then form into 4 patties.
2 Add the remaining oil to a pan, and cook the patties for 6-8 minutes, turning occasionally.
3 Rinse and dry the lettuce. Trim if necessary, and tear into small pieces. Peel the red onion, and cut into rings. Rinse and slice the cucumber. Rinse the tomatoes, remove the core, and slice.
4 Cut the buns horizontally and toast the cut sides until golden brown. Top the bottoms of the bun with lettuce, and spread with mayonnaise. Add the tomatoes, and top with mayonnaise. Place the burger over the top. Top the burger with the cucumber and onions. Top the hamburgers with the tops of the buns.
5 Serve with coleslaw, if you'd like.