Preheat the oven to 220°C/425°F/gas 7. Pick the sage leaves into a pestle and mortar, smash up with a good pinch of sea salt and black pepper, then muddle in 3 tablespoons each of olive oil and red wine vinegar. Rub all over the pork, then roll it up lengthways and secure with five bits of string. Sit it directly on the bars of the oven with a large shallow casserole pan beneath to catch the juices, and roast for 1 hour. Trim and quarter the fennel, reserving any leafy tops in a little bowl of water. Pull the pan out of the oven and toss the fennel into the pan juices. Reduce the temperature to 180°C/350°F/gas 4, and roast for 1½ hours, or until cooked through, shaking the fennel halfway.
Transfer the pan to a medium heat on the hob. Use tongs to move the pork and half the fennel to a platter, cover with tin foil and leave to rest. Mash or break up the remaining fennel in the pan. Boil the kettle. Stir the rice into the pan for 2 minutes, then add the saffron. Pour in the wine and let it cook away. Add some boiling kettle water, wait until it’s been fully absorbed, then add some more. Stir regularly, adding more water until the rice is cooked – around 20 minutes. Finely grate and beat in the Parmesan, season to perfection, adding more Parmesan, if you like, then loosen with water to an oozy consistency. Pop the lid on, turn the heat off and let it sit while you remove the string and carve or slice the pork, sprinkling with the reserved drained fennel tops. Serve with the risotto.