Season minced beef with salt and pepper. Mix well and make four patties.
Cook the meat patties on an oiled grill and brush with BBQ sauce.
Caramelised onion: heat oil and butter together in a frying pan. Add onion and saute till slightly brown. Add sugar and continue to saute till onion is crispy and brown.
Burger: Spread tomato sauce on the top half of the burger. Layer one beef patty, tomato slice, another beef patty, caramelized onion, cheese slice and lettuce (in that particular order) on the buttered half of the buns.