Heat the oven to Gas 7, 220°C, 200°C fan. Pull off the outer layer of fat on the lamb and discard.
To make the crumb mixture, blend the breadcrumbs, mint and the rind and juice of 1 lemon in a food processor until finely chopped. Spread this mixture over the lamb and place flat in a large roasting tin, crumb-side up. Squeeze the juice from the remaining lemon and add the juice, squeezed lemon halves, shallots, and garlic to the roasting tin. Roast for 45 minutes (or 25 minutes per lb) until the meat is cooked.
Remove from the oven. Arrange the lamb racks on a serving platter with the bones facing inwards and interlocking to make an arch. Cover with foil to rest for 10 minutes while you make the gravy.
Remove from the lemon halves, garlic cloves and any large shallot pieces from the roasting tin. Place the tin on the hob and add the butter. Heat until foaming, then add the flour. Cook 2-3 minutes until thickened, then pour on the red wine and 200ml boiling water, then add the redcurrant jelly and Worcestershire sauce. Simmer for 3-4 minutes then strain. Serve with the lamb.
If using smaller, trimmed, pre-packed lamb racks, you will need only half the crumb mixture.
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