Heat the oil in a saucepan and sauté the onion for about 5 minutes until it’s softened but not coloured. Add the stock and as soon as it is boiling, add the peas. Return to the boil and cook for 3 minutes. Stir in the mascarpone, basil and season with salt and pepper.
Blitz the soup in a food processor or blender or using a hand blender until smooth. Serve with some chunky bread.