Put all the ingredients in a large saucepan or stockpot and pour in 1.5 litres of cold water. Bring to the boil, then reduce the heat, half cover the pan and simmer gently for 1 hour, or until reduced by about one-third.
Melt the butter in a large stainless-steel or enamel saucepan. Add all the vegetables, except the lettuce, and stir until they glisten with butter, then cook over very low heat for about 15 minutes, or until the vegetables are just beginning to soften. Do not allow them to brown.
Add the lettuce, herbs, peppercorns, mace and water to the saucepan and bring slowly to the boil. Reduce the heat, skim any scum from the surface, partly cover the saucepan and cook on low heat for 2–3 hours.
Remove the pan from the heat, strain the stock and discard the vegetables. Leave to cool.
Store in a plastic container in the fridge for up to 4 days or freeze for up to 6 months.
Strain the stock. Set aside until cold, then refrigerate overnight. Next day, carefully discard the solidiﬁed fat from the surface. The stock can be frozen in measured quantities for later use.