Cook the peas in boiling salted water for 5-8 minutes or until tender (cooking time will depend on the size and age of the peas). Drain and refresh. Put the peas in a blender or food processor with the leaves picked from the marjoram. Blend to a purée. If the mixture is too thick to blend easily, add a few drops of stock or water. Season to taste with salt and pepper, then put back into the saucepan (or a microwave bowl).
Stir in the cream to enrich, if using, and reheat gently for serving.