Heat the oil in a saucepan, add the onion, garlic and thyme and cook over a low heat, stirring frequently, for 5 minutes until the onion is softened. Add the potatoes and cook, stirring frequently, for 5 minutes.
Stir in the peas, stock and salt and pepper. Bring to the boil, then reduce the heat, cover and simmer gently for 20 minutes.
Transfer the soup to a food processor or blender, add the rocket and lemon juice and process until smooth. Return to the pan, adjust the seasoning and heat through. Serve immediately, drizzled with a little extra oil.