Fennel and mortadella risotto

Chef Max Mariola inspired this tasty, unusual and inexpensive risotto, which I loved.
INGREDIENTS FOR 2
140 G CARNAROLI RICE
1 FENNEL BULB
4 MORTADELLA SLICES
HALF AN ONION
HALF A GLASS WHITE WINE
50 G BUTTER
EXTRA VIRGIN OLIVE OIL
4 WALNUTS
GRATED PARMIGIANO
PEPPER
SALTVEGETABLE STOCK: CELERY, CARROTS, ONION
I bring 1 carrot, a celery stalk and an onion in one and a half litres of cold water to the boil and cook for 20 minutes.I cut mortadella into strips and thinly slice fennel with a mandoline.
I shell walnuts for finishing.I put chopped onion, a little oil and 30 g butter in a large pan.I soften gently for about ten minutes until creamy, adding stock if needed.I add rice and toast a few minutes. I add white wine and let evaporate.I add fennel and stir.I cover rice with boiling vegetable stock, then gradually add more while stirring until cooked. I adjust salt.When rice is cooked but firm, I add mortadella and turn off heat.I add plenty of Parmigiano and ground pepper.I mix in remaining butter.I add stock if needed to keep loose.I plate and crumble walnuts on top.