Melt the butter in a saucepan and add the onion. Cook until softened. Sprinkle with flour and stir in. Add the stock, stirring rapidly. Add the chicken mince. Add salt and pepper, nutmeg and Tabasco. Remove from the heat and add the egg yolks, stirring rapidly.
Spoon the mixture into a square cake tin, cover with a piece of wax paper and refrigerate overnight.
Using your fingers, roll the mixture into 12 balls. Dust lightly with flour. Beat the egg and water together and dip the balls in it then coat with breadcrumbs. Chill meatballs until ready to cook.
Heat the deep frying oil and when hot cook the meatballs a few at a time. Cook 3 minutes or until golden. Serve with gravy.