Mix the mushrooms and bacon with the chicken, then stir in the sage, egg yolk and seasoning. Shape into 20 small meatballs and chill for 30 minutes.
Fry the meatballs in 1 tablespoon of the oil for 5 minutes until lightly browned all over, then transfer to a roasting tin and cook in a preheated oven, 190°C (375°F), Gas Mark 5, for 15 minutes.
Meanwhile, fry the onions in the remaining oil in the cleaned frying pan until softened and just beginning to brown. Sprinkle over the sugar and cook for 5 minutes more, stirring frequently until a deep brown.
Mix in the flour, then gradually mix in the stock, season and bring to the boil. Simmer for 2–3 minutes until thickened. Add the meatballs to the gravy and gently stir. Spoon into bowls and serve with mashed potatoes, peas and green beans.