Put 1 tbsp of butter in a cold, non-stick frying pan. Tip in the leek and cook gently until soft, then scrape out of the pan and add another tbsp of butter and a little oil. Pinch the sausage meat into the pan in small nuggets, and brown, but don’t cook through, then leave to cool. Bring a pan of salted water to the boil, add the diced potato and parsnip, and boil for 3-5 minutes until almost tender, but not soft. Drain and cool.