Preheat the oven to 220ºC.
Roughly chop the apple, leeks, potatoes and shallots, making sure nothing is too thinly sliced. Add the sausages to a deep baking tray and then add the chopped ingredients. Season with salt and pepper.
Mix the olive oil with 1 tbsp of the wholegrain mustard and pour over the ingredients in the tray. Mix well.
Roast for 25 minutes. Meanwhile, mix the veg stock with the remaining tbsp of wholegrain mustard.
After 25 minutes, remove the tray from the oven and move everything around. Pour the stock in, cover with foil and return to the oven to roast for a further 35 minutes, removing the foil for the last 5 minutes. Make sure potatoes are cooked through.
Top with the sage, serve and enjoy!