Remove the chipolata sausages from the fridge and let them come to room temperature to prevent cracks.
Remove the green stems from the fennel tubers. Then cut the tubers in two (from top to bottom, not bother) and remove the hard piece. Cut the fennel into small cubes.
Finely chop the onion and fry it in a little butter. Then add the fennel to the pan. Season with salt and pepper and let it cook gently.
Meanwhile, prepare potato puree according to the basic recipe. When the fennel is soft, mix it through the puree and stamp everything well again.
Heat a frying pan over medium heat and melt a knob of butter. Bake the chipolata sausages on both sides until they are nicely brown and cooked.
Serve the sausages with a generous scoop of fennel puree, a portion of fitted onions and a handful of lettuce or watercress.