Melt the butter in a large non-stick pan. Add the rice and garlic and stir over the heat until you hear the rice crackle.
Pour in the wine and allow it to bubble until it has been absorbed into the rice. Stir in the spring onions and a third of the hot stock.
Set the timer for 20 minutes, and allow the risotto to bubble, stirring every now and then, and topping up with extra stock as it gets absorbed. After 10 minutes add the courgettes, then continue cooking for the rest of the time until the rice is tender, and looks creamy, but the grains are still nutty.
Take from the heat, stir in the salmon with the mascarpone and lemon zest, season to taste then leave to rest for 5 minutes before serving.
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