Heat the olive oil in a large pan. Add the garlic and leeks and cook over a gentle heat until softened. Add the rice and cook for 2 minutes.
Gradually add the stock, one cup at a time, stirring well and allowing all the stock to be absorbed before adding the next cup. Continue until all the stock is absorbed, and the rice is cooked, about 20 minutes.
Remove rice from the heat, add butter, Parmesan, mascarpone, lemon juice and zest. Season and allow to stand covered for 5 minutes.
For the mussels, heat a large pot over medium heat, add the butter and sauté the spring onions for 2 minutes.
Add the wine, mussels and thyme, cover and cook for 5 minutes. Discard any mussels that are unopened.
To serve, divide risotto between serving bowls, then top with mussels and pan juices and garnish with fresh thyme.