1. Preheat oven to 190°C fan.
2. Trim the leg of lamb by removing the fell. The fell is a thin outer layer of fat that you may get if you buy an untrimmed leg from a butcher; it's very tough, so it's important to remove all of it, making sure to dry the lamb with a piece of kitchen towel.
3. Soften the butter, add the garlic, anchovies and rosemary and mix well. Add salt and pepper. Spread evenly around the whole leg of lamb. Place in a big ovenproof dish or baking tray and put it in the preheated oven for 45 minutes.
4. Mix the potatoes and onions well with salt, pepper and the rosemary.
5. After 45 minutes, take the lamb out of the oven and place the potato and onions in the ovenproof dish underneath and around the lamb, making sure to mix the potatoes with fat and juice from the bottom of the ovenproof pan. Divide the butter into small dollops and spread evenly on the potatoes. Place it back in the oven and roast for another 25 minutes.
6. Snap off the lowest third of the asparagus, cut into pieces and mix with the radishes. After the 25 minutes, mix the asparagus and radishes into the potatoes and onions in the oven and roast for another 5 minutes.
7. To make sure the lamb does not overcook, use a thermometer to see when it is done. The temperature in the center should be 66°C for a medium roast.
8. Serve the lamb with the vegetables.
1. Make sure to spread the butter mixture evenly around the leg of lamb, the butter spread makes it tasty and crispy.
2. While the lamb is in the oven take it out now and then, and with a spoon pour over the lamb again some of the melted butter and juices that has run off into the ovenproof dish.
3. For variation, you can use thyme or sage instead of rosemary.