Work the meat thoroughly with the onion, rice, herbs, salt and pepper. When thoroughly mixed, shape into walnut-sized balls and roll lightly in flour.
Bring the stock to the boil in a shallow pan and drop in the meatballs one by one. Cover loosely and simmer gently for 1 hour.
Just before you are ready to serve, make the sauce. Whisk the egg whites in a roomy bowl. When they are