Preheat the oven to 200°C.
Peel and grate the potatoes with a coarse cheese grater, then put this in a pan of boiling water for 3-5 minutes. Drain and cool.
Thinly slice the onions and mushrooms, fry them in the oil until soft, then transfer to a baking tray. Put the sausages in the pan, brown them for about 5-8 minutes all over then put them on top of the onion and mushroom mixture.
Mix the mustard with the honey, then pour this over the sausages and combine.
Season the grated potatoes with salt & pepper, then add a Tbsp of oil and combine well.
Cover the sausages evenly with the potatoes.
Bake for 25-30 minutes, until the rosti is golden and crispy.
Serve warm.