Heat one tablespoon of oil in a pan, add chopped onions and sauté till golden. Add ginger paste and minced garlic and stir. Add salt and mix. Add chopped green chillies, garam masala powder and stir-fry for a minute. Remove from heat and add this mixture to the chicken mince. Adjust salt and add garlic pepper powder and mix well. Add chopped prunes and mix well.
Shape the mixture into four patties. Heat one tablespoon of oil in a fry pan, add the onion roundels and cook till they are a bit soft. Drain and keep aside. Heat some oil in the fry pan, place the chicken patties and cook on medium heat turning sides till both sides are cooked and golden.
Drain onto an absorbent paper. Cut the burger bun into two and warm them on a pan or griddle. Place the bottom part of the heated bun on a plate. Apply some mustard sauce and place lettuce leaves. Place the chicken patty on the lettuce leaves and top it with the sautéed onion roundels. Cover it with the top half of the bun. Serve immediately.