Take the chipolata sausages out of the fridge and let them come to room temperature to prevent them from bursting.
Remove the green stalks from the fennel bulbs. Then cut the bulbs in half (from top to bottom, not across) and remove the hard part. Cut the fennel into small cubes.
Finely chop the onion and sauté it in a bit of butter. Then add the fennel to the pan. Season with pepper and salt and let it cook gently until tender.
Meanwhile, prepare the mashed potatoes according to the basic recipe. When the fennel is soft, mix it into the mash and mash everything together well again.
Heat a frying pan over medium heat and melt a knob of butter. Fry the chipolata sausages on both sides until they are nicely browned and cooked through.
Serve the sausages with a generous scoop of fennel puree, a portion of pickled onions, and a handful of lamb's lettuce or watercress.