Peel the potatoes and cut them into chunks. Cook them in salted water until just submerged, 20 minutes. Cut the broccoli into florets and the stalk into small pieces and cook them in a separate pan, in plenty of salted water, for 6-8 minutes until al dente.
Meanwhile, roughly chop the spring onions and cut the shallots into wedges. Heat a dash of olive oil in a frying pan and fry the spring onions and shallots until golden brown. Remove from the pan and brown the sausages for about 6 minutes. If necessary, add the spring onions and shallots again to warm briefly. Keep the sausages, spring onions and shallots warm under a lid or on a plate with aluminium foil.
Drain the potatoes and mash them with the mascarpone and goat's cheese until smooth and creamy. Season with salt and pepper. Spoon in the broccoli florets, shallot and spring onion and serve with the sausages. Chop the chives and sprinkle over the mash, garnish with capers and some more goat's cheese.