After shelling the peas, save the pods, wash them and put them to boil in the ham stock while preparing the soup. Heat the butter in a large saucepan and soften the onion in it, then add the lettuce, mint and parsley. De-rind and chop the bacon. Fry it for about 2 minutes, turning it from time to time; add to the saucepan with the peas, salt, pepper and a small amount of sugar. Strain the stock and add. Bring to the boil, stirring, then simmer for about half an hour until the peas are quite soft. Sive or liquidize, taste for seasonings and add a little milk or cream if needed (but not too much, for the fresh flavor must be preserved). Garnish with chopped parsley or mint.