Heat the butter in a small frying pan over a low heat with the thyme and 1/2 tsp freshly ground black pepper until the butter begins to foam. Remove from the heat.

Bring a large pan of salted water to the boil, tip in the carrots and swede and simmer for 30-40 mins until tender. Mash well. While still hot, stir through the butter and serve. Will keep chilled for up to two days or frozen for up to a month.