1. Tear the bread into small pieces, cover with water and set aside to soak.
2. Grate the onion on the fine teeth of a cheese grater. Tip the onion pulp and juices into a big mixing bowl and add the mince, mayonnaise, garlic and herbs.
3. Squeeze all the water from the bread and add it to the bowl. Season the mixture to taste with salt and pepper.
4. Using your hands, knead the ingredients together until very well combined. Roll into balls about the size of an apricot.
5. Heat the oil in a large, shallow, non-stick pan and gently fry the meatballs on all sides for about 8 minutes, or until golden brown. Watch them closely, as they can burn quickly. Remove with a slotted spoon and set aside on a plate.
6. To make the sauce, add the chopped onions to the same pan and cook gently until soft and a deep golden brown. Add the mushrooms and herbs fry for a further 5 minutes.
7. Sprinkle over the flour and garlic, stirring well to coat the vegetables. Whisk in the stock, then bring to the boil, stirring constantly to prevent lumps forming. Cook over a medium-high heat for 5 minutes, or until slightly thickened.
8. Return the meatballs to the pan, turn down the heat and simmer for 10 minutes.
9. Push the meatballs to one side and scoop out half a cup of the sauce. Place in a separate bowl and whisk in the mayonnaise and mustard.
10. Return this mixture to the pan, then stir and shake the pan gently so the meatballs are evenly coated. Simmer for a further three minutes, sprinkle with lemon juice and garnish with fresh parsley.