Heat the oil in a pan. Sauté the spring onion and fennel. Add the rice and lemon zest, sauté while stirring until the rice is translucent.
Pour in the wine and the reserved lemon juice, reduce completely. Gradually add the stock, stirring frequently, so that the rice is always just covered with liquid, simmer for approx. 20 mins. until the rice is creamy and al dente.
Mix in the butter, cheese and reserved spring onion greens, season.
Scrub the mussels under running water, remove the beard, discard any mussels that are already open.
Heat the oil in a pan, sauté the spring onion and the reserved fennel for approx. 3 mins.
Add the mussels, pour in the wine, cover and cook for approx. 5 mins. until the mussels open, season with salt, strain. Discard any mussels that are still closed – these are inedible.
Serve the mussels with the risotto.