Bring the shallot, white wine, and sorrel stems to a boil. Reduce the mixture by half, then add the vegetable or veal stock. Stir in the creme fraiche, reduce the heat, and simmer until thickened.
Coarsely chop the sorrel leaves, then puree with a little of the sauce. Add back to the pan, and stir until combined. Season to taste with salt and pepper.