1) In a large saucepan boil some water and add a little salt. Add the carrots and swede, turn down the heat, cover and simmer gently for 20-30 minutes. Check the swede is soft as it takes longer to cook than the carrots.
2) Once soft, drain well in a strainer, put back into the pan and blend with a stick blender.
3) Add the butter and the mustard.
4) Season to taste and blend until smooth.
Top tip: Turn any left-over mash into a winter warming soup by adding vegetable stock, a chopped, fresh sage leaf and a little creme fraiche. It can also be added to risotto for a rich smooth texture and a real nutritional boost.