Add potato, onion and garlic to a pan. Pour over 800ml of the stock, cover with a lid and simmer for 20 minutes.
Add peas, mint, and ¼ tsp salt. Simmer for a further 5 minutes.
Place everything into a blender or use a hand processer to blend until smooth.
Pop back into the pan over a low heat and stir in the yoghurt. Add additional salt and pepper to taste if needed.