Heat the butter in a saucepan and add the fennel, leeks and potato. Cook over a medium heat for about 5 mins, turning the vegetables over in the butter. The vegetables shouldn’t colour. Add a splash of water, cover the pan and cook the vegetables for 20 mins, stirring every so often. Add the stock or water and garlic, season, bring to the boil, then turn the heat down low and cover the pan with a lid or foil.