Peel the carrots and potatoes and cut into small cubes. Bring the cubes to the boil with the cream and milk. Simmer gently until the liquid has boiled down (be careful not to burn anything - the transition comes suddenly!).
Puree with a magic wand. If you like it a bit grainy, you can skip the step of passing the puree through a sieve. Now stir in the butter and season with the spices. Be careful with nutmeg!