Peel the shallot and mince it finely. Remove the stems from the sorrel leaves, then stack the leaves on top of each other, and roll them up from side to side, like a fat green cigar. Slice this crosswise in very thin strips.
Heat a skillet over medium heat, and when it's hot, put in a knob of butter to melt. Add the minced shallots and sweat them – that is, cook them without letting them get brown – until they are soft.
Add the strips of sorrel, and cook them until they are thoroughly wilted. Don't be disconcerted when the sorrel turns a sort of swampy green color, this is normal.
Add the wine, and bring to a boil. Let it bubble and cook until the liquid is reduced to about half its volume.
Add the cream, and lower the heat. Don't let it boil, but cook the sauce gently until it will coat the back of a spoon.
Notes
Sorrel sauce will keep, covered and refrigerated, for about 3 days. Don't let it go to waste!