To reheat in the oven - Place in an oven safe dish then pour over 120mls / half a cup of soy milk and a few tablespoons of vegan butter (no need to melt it, lumps of it are fine). Don't stir. Leave it pooling on the top. This will protect the potato and stop it from drying out. Cover tightly with a lid or foil and bake for about 20 to 25 minutes at 350 °F (175 °C) or until piping hot. Give the mash a good beat with a wooden spoon or a handheld mixer before serving to make it nice and fluffy.  To reheat on the stovetop - Transfer the mashed potatoes to a large pan and add half a cup of milk and a few tablespoons of vegan butter. Place the pan over low heat and stir constantly until piping hot.  To reheat in a slow cooker (or to keep warm after mashing) - Transfer the mashed potatoes to the slow cooker insert and set to low. They should be hot after about 2 hours but can be left on for up to 4 hours. Before serving add half a cup of warm soy milk and a few tablespoons of vegan butter then beat well with a wooden spoon.