Take the lamb out of the fridge 1 hour before cooking so it comes up to room temperature.
Preheat the oven to 220°C / 200°C fan.
Using a sharp knife, make small incisions all over the lamb. Push the slices of garlic into the cuts.
In a small bowl mix the rosemary, lemon zest, Dijon mustard, olive oil, a generous pinch of salt and plenty of black pepper.
Rub this mixture all over the lamb, pushing it into the cuts so the flavour gets right into the meat.
Scatter the chopped onion in a roasting tray and sit the lamb on top.
Roast for 20 minutes to brown the outside.
Turn the oven down to 180°C / 160°C fan and roast for another 50–60 minutes for pink lamb (internal temp about 55–60°C).
Transfer the lamb to a board and leave it to rest for at least 20 minutes before carving.