Cut the peeled potatoes and unpeeled pumpkin into cubes of approximately the same size. Cook both in not too much salted water. Drain, keeping some of the cooking water in the pot. Mash the pumpkin and potato pieces well (for a slightly coarser puree) or pour them through the brisk Lotte (for a finer puree). Heat the milk (e.g. in a microwave) and stir into the potato and pumpkin mixture. Stir in butter and season to taste with salt, nutmeg, chilli, pepper and thyme.