Peel and chop the onion. Peel, wash and dice the potatoes. Melt butter in a pot, fry onion and potato. Set aside 1/4 of the peas. Add the remaining peas to the pot and sauté briefly.
Deglaze with stock, bring to the boil and simmer covered for about 10 minutes. Meanwhile wash coriander, dab dry, remove leaves and chop coarsely. Set aside some coriander for sprinkling. Add the remaining coriander to the soup 1-2 minutes before the end of the cooking time.
Puree the soup very finely with the cutting stick, add the remaining peas to the soup and bring to the boil. Season to taste with lemon juice, salt, pepper and sugar. Arrange soup in bowls and sprinkle with remaining coriander.