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Mix the balsamic vinegar, 1 tsp olive oil and rosemary together in a bowl. Add the lamb steak, ensuring that it is completely covered in the marinade, and set aside.
Boil the potatoes until tender. Lift out of the pan with a slotted spoon, keeping the pan of water simmering. Heat 2 tsp of the oil in a frying pan, add the potatoes and garlic, and fry gently until golden.
Sprinkle a little salt over the potatoes, then push to the side of the pan. Remove the garlic. Season the lamb and fry alongside the potatoes for 1-2 mins each side, depending on the thickness of your steak. Remove the lamb and potatoes and leave to rest in a warm place.
Keep the pan on the heat and add the stock. Bring to the boil, scraping any sticky bits from the bottom of the pan, and let it bubble until reduced by half. Stir through the redcurrant jelly until melted. Taste and season, adding a dash of balsamic vinegar if it is too sweet.
Meanwhile, blanch the beans in the simmering water for 3 mins, then drain. Return to the pan and stir through the butter, mint and seasoning.
Serve the lamb with the potatoes and beans, and the sauce spooned over.