Place the chicken breasts between sheets of cling film and, using a mallet or rolling pin, beat lightly until about 1.5cm thick. Remove the cling film and season well.
Mix together the lemon zest, chopped rosemary and garlic and spread over the chicken breasts.
Preheat the grill to medium and line the grill pan with foil. Beat the egg white on a plate with a little salt and pepper until frothy. Dip the chicken in the egg white and place on the grill pan. Grill for 10-12 minutes, turning once until browned and crisp.