Place the garlic, rosemary, salt and pepper into a mortar and crush it with the pestle until you get a paste-like consistency then add the olive oil and lemon zest, stir to combine and set aside. (You can also do this in a food processor.)
With a sharp knife score the lamb, just don’t cut too deep, then rub the rosemary and garlic mixture all over the lamb, making sure you get into the cuts.
Put the lamb on a rack placed over a baking dish and bake for about 90 mins, the timing all depends on the size of the lamb, and how you like it cooked. Lamb should be a little pink but we like ours done just a little bit more. The lamb I used weighed close to 2kg.
Serve with your favourite vegetables, I love broadbeans so I used them in place of our usual peas but one of them has to be potatoes, baked, roasted, fried, whatever.
The lamb can be prepared a day in advance, just do steps 2 and 3, cover and place in the fridge. Next day take it out and leave at room temperature for about 30 mins before placing in preheated oven.
Use leftover lamb in sandwiches or maybe a shepherds pie.