Place the butter in a saucepan over a medium heat. Once the butter has melted stir in the onions, potatoes and garlic – season with sea salt and freshly ground black pepper and stir. Reduce the heat to low and cover with a lid. Every couple of minutes stir and check to make sure the vegetables aren’t sticking to the bottom of the pan. Leave to cook for five minutes.
Pour in the hot stock, leave the lid off and allow to cook for about 15 minutes or until the potato is completely cooked. Then pour in the frozen peas and cook for three minutes, followed by the fresh mint and cook for another minute.
Next blend the soup in a food processor and taste for seasoning.