For the braised lamb shank, preheat the oven to 150C/300F/Gas 2.
Heat the oil in a heavy-based frying pan and season the shanks with a little salt, then add them to the hot frying pan. Colour all over as much as possible, and, once coloured, add to a casserole.
Tip the onion and celery into the frying pan and cook until softened. Add the vegetables to the casserole. Pour in the stock and add the bay leaves. Bring to the boil and skim off any impurities that rise to the top of the pan.
Put a lid on the casserole and braise in the oven for 1½-2 hours.
Remove from the oven and leave to rest in the braising liquor.
Measure out 500ml/18fl oz of the braising liquor into a pan and cook until the liquid has reduced to a glaze (or sauce) consistency.
For the borlotti beans, using a knife, peel the lemons and cut the peel into fine strips (like marmalade). Blanch the peel in a pan of boiling water and refresh in a bowl of ice cold water three times.
Juice the lemons and place in a saucepan with the sugar. Bring to the boil, then add the peel and cook until it becomes a light syrup.
Heat a large frying pan and add the oil. Once hot, add the onion and fry with the mergeuz sausage until lightly coloured. Add the garlic and cook for a further two minutes.
Remove the sausages and slice into bite-sized pieces.
Add the beans to the pan with half of the reduced braising liquor, the lemon syrup and most of the picked flatleaf parsley.
To serve, heat the tablespoon of olive oil in a pan and fry the garlic until crisp. Drain on a plate lined with kitchen paper.
Divide the beans between two plates, lay the braised shanks on top and brush the shanks with the remaining glaze. Garnish with the remaining parsley, one of the bay leaves each and the crispy garlic.