Put the stock in a large saucepan. Stir together the leek, carrot, and parsley. Add the ground chicken, egg whites, and 2 tablespoons water. Pulse to mix well. Stir this mixture into the stock. Bring the stock very slowly to the boil, stirring constantly. The egg mixture will congeal and form a “raft” on the top of the stock, collecting all the impurities from the stock. As soon as it begins to boil, stop stirring and turn the heat down to low. A hole will have formed on the top of the “raft”. Make it slightly larger with a spoon, then simmer the stock very gently for 45 minutes. Remove from the heat and let stand 5 minutes.
Line a sieve with a double layer of cheesecloth. Using a skimmer, lift off as much egg white from the top of the consommé as you can, and discard. Ladle the consommé into the lined strainer and let it drip through in its own good time. Don’t be tempted to press it or you’ll make it cloudy. When it has all dripped through, season and taste the consommé. Serve hot or cold with the garnish of your choice.