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Cook the kale in boiling water until just wilted, then refresh in iced water. Drain and carefully squeeze the excess water from the kale, then roughly chop. Tip into a mixing bowl, stir in the garlic, anchovies, zest and breadcrumbs, and season (add lots of pepper). Pack the stuffing into the cavity of the lamb and tie with string – it doesn’t matter if it’s messy, just make sure the filling is secured. Can be prepared up to a day ahead and kept in the fridge.
Heat oven to 190C/170C fan/ gas 5. Mix the herbs with the hay (if using), transfer to a deep roasting tin, then pour over the stock. Nestle the lamb on top, drizzle with oil and sprinkle with salt. Roast for 1 hr 20 mins for pink lamb, adding 10 mins if you want it cooked through and another 10 mins for well done. Rest for 20 mins, then carve into thick slices and serve.