Preheat the oven to 120°C (100°C fan, gas 1). Cut the lamb into four equal pieces. Wash the rosemary, shake dry and tie some to each piece of meat with kitchen string.
Thoroughly clean the mushrooms. Halve the large ones; leave small ones whole. Drain and dice the tomatoes.
Chop the olivess. Peel and finely dice the onion and garlic.
Fry the lamb in a non-stick pan with 1 tbsp oil, turning to brown on all sides. Season with salt and coarsely ground pink pepper. Place on the rack set over a roasting tin and cook for 10 minutes in the oven.
Saute the onions, garlic and chanterelles in a pan in the remaining oil for 5 minutes. Add the tomatoes and olives. Pour in the red wine and stock and season with salt and pepper. Simmer for a few minutes and seaosn to taste.
Remove the lamb from the oven, cut off the string and carve into slices. Serve with the vegetables.