Clean the leeks, cut lengthwise, wash and finely chop. Peel garlic, chop. Rinse thyme in cold water and toss dry. Strip off the leaves. Wash lemon, grate peel. Squeeze juice. Cut shrimp into pieces, mix with lemon juice and zest. Heat butter in a pot. Sauté leeks, garlic and thyme in it. Mix in rice. Add a ladleful of stock and let it evaporate. Mix remaining stock with salt, cayenne pepper and saffron and gradually add to rice. Cook rice uncovered over medium heat for about 25 minutes, stirring very frequently. When rice is cooked but still “al dente” like pasta at the core, wash, dry and finely chop parsley. Wash and clean the spring onions and cut into rings. Mix shrimp with mascarpone into the rice and continue cooking for a few minutes until the shrimp have turned red. Season again and serve with parsley and onion rings taken care of.