Peel and thinly slice the beetroot, place in a large saucepan, add the stock, vinegar, garlic, spices, herbs and 500 ml (17 fl oz/2 cups) water and bring to the boil. Reduce the heat to low and simmer for 2 hours. Strain, discarding the solids. Return the liquid to the pan and simmer until reduced to 500 ml (17 fl oz/2 cups). Season with salt. Add the agar agar and cook, stirring continuously, until dissolved. Transfer to a metal bowl and chill over ice until set. Transfer the gel to an upright blender and blend until smooth. Pass through a fine sieve and set aside.
This can be made 1 day in advance and stored in an airtight container in the fridge.