Apparently sorrel can be cooked just like spinach, but my favorite way to use it is in soup.There are a number of recipes out there, mostly French, but including at least one Russian one that I haven't tried yet. Below are the very simple ingredients for the easy-to-make version we like. The key thing to remember is that you don't actually "cook" the sorrel leaves. Putting them in at the end keeps both the color and flavor bright and fresh.
2 cups sorrel leaves
2 T. butter
1/2 large or 1 smallish onion, chopped
1 potato (to yield 1 cup cubed)
4 cups broth (chicken or vegetable)
1/4 to 1/3 cup cream (fatfree half-and-half is fine)
1 T. sherry
salt and pepper to taste
In at least a 3-quart kettle, sauté the chopped onion in butter until translucent. Add potato and broth and simmer 20 minutes, until the potato cubes are very tender.