Place the pan over a medium heat. Once hot, add the butter.
When the butter has melted, add the onion and cook for 5 mins, stirring often, until translucent.
Add the garlic and cook for a further 1-2 minutes until fragrant.
Tip in the flour and cook, stirring continuously, for 1-2 minutes then add all the remaining ingredients.
Stir thoroughly then increase the heat and bring up to the boil.
Once boiling, reduce the heat and simmer for 35-40 minutes until the soup has reduced by half. Stir occasionally (especially towards the end of the cooking time) to prevent it catching and burning on the bottom.
Allow to cool before blending with a stick blender or in a food processor.
Bring back to the boil and serve immediately or cool completely and freeze for another day. (Defrost completely before use and consume within 3 months.)