Sauté the onion in the butter. Add the barley and carrots and sauté briefly, turning. Extinguish with white wine and cook while stirring.
Add a part of the soup – so that the barley is well covered with water – , cook gently on a low fire with the lid closed, stirring occasionally, adding soup if necessary (detailed as in a normal risotto).
After half an hour add the leek and let it simmer for 15 to 20 minutes with the lid closed.
Afterwards, the barley grains should still have a slight bite and be very wet, similar to a risotto.
Stir in the meat together with the mascarpone and just heat well. As a last step add cheese, season strongly with salt and freshly ground pepper.