In a large saucepan or Dutch oven, heat the oil and butter and sauté the onion or leek, sprinkling with salt, for a few minutes, or until soft. Stir in the herbs.
Add the stock, bring to a simmer, then add the lettuce and simmer for 5-10 minutes.
Add the cream and purée using a hand-held immersion blender right in the pot, or carefully transfer to a blender to purée until smooth.
Serve topped with small croutons, if you like.