Fry lamb bones in oil for 10 minutes, then add onion, celery, carrot, thyme, and bay leaf.
Add water, bring to a boil, then simmer for 45 minutes while skimming foam.
Strain the stock through a fine sieve and remove surface fat once cooled.
Fry bread cubes in oil until golden brown and place in a mixing bowl.
Sauté sliced leek, onion, and garlic until softened and add to the bread.
Stir in capers, lemon zest, mint, and parsley, then season and cool.
Crush mint and sugar together, then stir in white wine vinegar and olive oil.
Flatten the lamb to a 3cm thickness using a mallet and season.
Spoon stuffing onto the lamb, roll it to enclose, and truss with string at 3cm intervals.
Weigh the lamb and place it into a roasting tin.
Sauté onion, thyme, and garlic for the potatoes until softened and browned.
Grease a 20cm x 30cm roasting tin with oil.
Layer potato slices and onions in the dish, finishing with a potato layer.
Pour stock over the potatoes and garnish with extra thyme.
Preheat oven to 200°C and roast lamb for 20 minutes per 500g, with potatoes at the bottom.
Remove the lamb and rest it covered in foil; keep the roasting tray for the jus.
Move potatoes to the top of the oven and cook for 15 minutes until browned.
Degrease roasting juices and whisk in flour over medium heat for 15 seconds.
Gradually whisk 300ml of reserved stock into the pan until smooth.
Simmer with redcurrant jelly and mint for 10 minutes, then season and strain.
Carve the lamb and serve over potatoes with jus and mint sauce.