METHOD

1. Combine mince, sauce and mustard in a bowl and season with freshly ground black pepper. Use clean hands to mix until very well combined. Divide mixture into four portions and shape into round flat patties.
2. Spray a large non-stick frying pan with oil and heat over medium-low heat.
3. Wait until cooked through on first side before turning to second side to minimise handling.
4. Add onions and cook, stirring often, for 10 minutes or until soft and golden brown. Transfer to a plate and cover with foil to keep warm.
5. Trim mushroom stalks. Heat a chargrill pan or barbecue flat plate over medium-high heat. Lightly spray mushrooms with oil and cook for 2 minutes each side or until tender and golden. Divide between serving plates.
6. Add burger patties to the chargrill and cook for 3 minutes each side or until cooked through.
7. Top mushrooms with lettuce, patties, cheese, tomato and onion.