Place the potatoes in a large saucepan and cover with cold water. Bring to the boil over a medium heat, then reduce to a simmer and leave to cook until just tender, 15–20 minutes.
Meanwhile, make the sausages. Put the walnuts, bread and onion into a food processor and pulse briefly to break them down a bit, then add the paprika and garlic powder and process in bursts, until you have a fairly smooth mixture that holds together. Season with salt and pepper to taste. Form the mixture by hand into 8 sausages.
Heat enough olive oil for shallow frying in a frying pan (skillet) over a medium heat and fry the sausages until lightly browned and crisp, 7–10 minutes. They need to be flattened slightly as you cook them and turned frequently so that they get nicely crisp all over.
Once the potatoes are tender, drain through a colander and leave to steam dry for a couple of minutes. Once dry, return the potatoes to the pan with the butter and milk, then cover with a lid and leave for a couple of minutes until the butter has melted. Mash the potatoes until smooth and creamy, then season to taste.
Place a generous spoonful of mash in the center of four plates, then divide the sausages between the plates and spoon over the onion gravy.
Serve hot.