Step 1

Use a sharp knife to cut 20 evenly spaced slits, about 2cm deep and 1cm long, into the top of the lamb. Cut each sprig of thyme into 3 even pieces (each piece should be about 3cm long). Insert a piece of thyme in each slit.

Step 2

Combine the chopped thyme and 1 tablespoon of the oil in a small bowl. Place the lamb in a large glass or ceramic ovenproof dish. Use your hands to rub the thyme mixture all over the lamb. Cover with plastic wrap and place in the fridge for 6 hours or overnight to marinate.

Step 3

Preheat oven to 180°C. Combine sweet potatoes, onions, salt and 1 tablespoon of remaining oil in a large roasting pan and toss gently. Place lamb on top of the vegetables. Insert a slice of garlic into each slit with the thyme.

Step 4

Whisk the remaining oil and lemon juice together in a small bowl and drizzle over the lamb. Season the lamb with pepper. Roast in preheated oven, basting with pan juices occasionally, for 1 1/4 hours for medium or until cooked to your liking. Turn the oven off. Transfer the lamb to a plate, cover loosely with foil and set aside for 10 minutes to rest. Return the vegetables to the oven to keep warm.

Step 5

Slice lamb across the grain and serve immediately with the vegetables.