1 Peel onion and finely chop. Saute in 1 tablespoon hot oil. Let cool slightly and then mix with egg, thyme and meat. Add breadcrumbs as needed until mixture is easy to shape. Season with salt and pepper and form 4 patties.
2 Cook in a hot pan in the remaining oil until golden brown, about 6-8 minutes, while turning occasionally.
3 Rinse lettuce, trim, shake dry and pluck small. Peel red onion and cut into rings. Rinse tomatoes, cut away stalks and thinly slice. Mix mayonnaise with mustard until smooth.
4 Cut burger buns horizontally and toast cut surfaces until golden brown. Brush bottom with mayonnaise mixture and top with lettuce. Add a hamburger patty, tomato slices and onions. Place top bun on and serve.