Combine the balsamic, oil and pepper, coat the meat, cover and marinate for approx. 30 mins. Heat the oil in a frying pan. Salt the meat and brown all over for approx. 8 mins. over a medium heat. Remove, transfer to a tray lined with baking paper.
Bake for approx. 10 mins. in the centre of an oven preheated to 150 °C. Switch off the oven and leave the lamb racks resting in the oven for approx. 10 mins.
Heat the oil in a pan and sauté the onion. Add the rice, sauté while stirring until translucent. Pour in the wine and reduce completely. Gradually add the stock, stirring frequently, so that the rice is always just covered with liquid, simmer for about 20 mins. until the rice is creamy and al dente. Stir in the Parmesan and thyme. Mix the Roquefort into the risotto, season. Carve the meat, plate up with the risotto.