Clean the fennel, eliminating the stems and the two outer sheaths. Cut them with a kitchen knife in half, wash, and cut into thin slices.
In a saucepan, heat the oil and put the finely chopped onion. Sauté gently without browning, then raise the heat and add the fennel and salt. Stir and let it simmer for about ten minutes.
Add the rice, stirring constantly. Let it cook for 3 to 4 minutes.
Add the wine and once most of it has evaporated, start adding the broth, one ladle at a time.
Cook the risotto al dente.
Remove from heat and stir in the butter and the grated Parmesan cheese.
Cover and let it rest for a minute or two before serving.