CHOP - Peel the carrots and sweet potatoes and roughly chop into large chunks. Leave the butter to soften at room temperature. BOIL - Put the vegetables into a large saucepan, cover with water and bring to the boil. Turn the heat down and simmer for 15 minutes or until they are soft when pricked with a fork. MASH - Drain and mash the carrots and sweet potatoes. Add the butter and mix thoroughly. Season with salt, a grind of black pepper and a grating of nutmeg.