Butterflied Roast Lamb with Tomato Gravy

Cheryl Potts gave me this recipe. It is the yummiest roast lamb I've ever tasted. And it's very easy to do.
butterflied leg of lamb (you can do this yourself by cutting it away from the bone in one piece - just have a go, you'll be fine!)
4 or 5 tomatoes
4 garlic cloves, thinly sliced
6 big sprigs rosemary
salt and pepper
oil
2 large tablespoons butter

1) Oil and salt the fatty side of the lamb. Sear 5 mins a side on the BBQ, fatty side first. Do not move it around - just leave it until it has a good colour on each side.
2) Place lamb in roasting tray, skin side down. Scatter garlic, tomatoes and rosemary. Add generous amount of black pepper, bit of salt, and drizzle lots of olive oil.
3) Seal with foil (Fold 2 strips together to make a piece wide enough - it needs to be air tight. Do this by placing them dull sides together, then roll/fold one side over about 2 cm, three times. Open up and it will be one giant piece of foil.) Bake at 180-200'C for 30-40min, or until completely cooked through and tender. Rest, covered with the foil.
4) Reduce juices etc. for 15 minutes or so in a saucepan. Can add a splash of red wine. When well reduced, push through a sieve and add butter.
5) Carve lamb and serve. Lamb will also be good carved and served cold in summer.