Sauté onions and leeks until translucent. Add lettuce and toss until wilted.
Add sugar, stock, salt and pepper. Bring to a boil and simmer for 10 to 15 minutes.
Purée and strain, then return to a boil. Add peas and simmer 5 minutes.
Combine crème fraîche and heavy cream, then whisk into soup.
Reheat without boiling. Adjust seasoning.