Heat the stock in a saucepan over a low flame, keep warm.
In a large skillet, over medium heat, sauté the onion in half of the butter until translucent. Add the sausage and break apart with a fork. Cook until lightly brown. Mix in the fennel and cook for a few minutes. Add the rice and stir together until opaque and lightly toasted. Add the wine and allow it to absorb. Add a ladle of the stock. Continue to cook and stir (preferably with a fork) until the liquid is almost fully absorbed. As the liquid absorbs, add more stock, a ladle at a time, waiting until almost completely absorbed before adding more. Cook until rice is al dente, about 15 minutes, being careful that the rice remains al dente.
Mix in the remaining butter and cheese. Remove the skillet from the heat. Serve immediately, garnished with s few fennel fronds.