CUT THE SORREL LEAVES into strips, melt half the butter in an omelet pan and cook the leaves until they have wilted and turned a gray-green color. This will take just a minute or two. Lightly beat the eggs with the water, season with salt and pepper, then stir in the sorrel. Melt the remaining butter in the omelet pan and when it is hot, add the eggs. As the edges cook, pull them into the middle of the pan with a fork, tilting the pan as you do so that uncooked eggs will flow into its place. When the eggs are cooked, fold the omelet in thirds and turn it out onto a plate. Return the pan to the fire and add the cream. Bring it to a boil and let it reduce enough just to thicken slightly. Make a slice down the center of the omelet, pour in the cream and serve.