Heat the butter in a heavy-bottomed saucepan and cook the onions in it gently, until they have softened but have not browned. Add the tomatoes and cook for a few minutes. Pour over half the boiling water, bring back to the boil then lower the heat and simmer for about 30 minutes. Remove from the heat and blend the soup in a liquidiser.

Boil the oatmeal in the remaining water for 10 minutes, then pour in the blended onions and tomatoes. Return to the heat and cook for 20-25 minutes. Season to taste and serve hot.