Heat the grill.
Slice the tops off 2 of the bread rolls and halve the rolls. Soften the third roll in lukewarm water, then squeeze the water out.
Mix the mince with the mustard, salt and ground black pepper, the egg and the softened roll. Form the mixture into 4 hamburgers. Heat the butter in a pan and fry the hamburgers until brown, turning several times.
Place a slice of cheese on each burger and put under the hot grill to melt the cheese.
Toast the bread roll halves quickly under the grill. Place one cheeseburger on each half roll and top with a few avocado wedges.
Garnish with beetroot strips and cress. Serve at once.