In a small saucepan, bring the shallot and water to the boil. Reduce the heat to a gentle simmer and skim the froth. Very gently simmer for 35 minutes or until the stock tastes sweet.
Add the chorizo and very gently simmer for a further 35 minutes or until reduced to 250 ml (9 fl oz/1 cup), skimming the stock of all fat and impurities throughout this time.
Pass through a sieve lined with muslin (cheesecloth), discarding the solids, and cool to room temperature. Check the seasoning and if it’s too strong or salty, add a little water.
Pour the stock into a 1 litre (35 fl oz/4 cup) capacity jug. Insert a hand-held stick blender into the stock but ensure there is no air trapped underneath the blades. Simply tilt the stick blender to the side to release any air as you don’t want to draw any air into the stock and create bubbles. Return the stick blender to an upright position, with the blades firmly on the base of the jug and blend the stock on the slowest setting — this will help prevent the stock ‘whirlpooling’ and drawing in air. While blending, add the xanthan gum very slowly, a little bit at a time, using a parfait spoon (long-handled teaspoon), to avoid the xanthan clumping. It’s best to just very gently tap the spoon on the side of the jug while blending. Add the lemon juice, adjust the seasoning and set aside at room temperature until needed.