Put the onion and potato in a large saucepan, along with the stock cubes, dissolved in 1.1 litres of boiling water. Bring to the boil, then reduce the heat and simmer for about 10 minutes, stirring occasionally.
Reserving a few mint leaves for the garnish, add the rest to the pan along with the frozen peas. Bring to the boil, then reduce the heat and simmer for 5 minutes.
Leave the soup to cool slightly, then transfer to a food processor or blender and blend until smooth.
Return the blended soup to the pan to heat through. Divide between 4 serving bowls and serve garnished with a swirl of yogurt and the reserved finely chopped mint.
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