I love sorrel's lemony, tart taste. Here's a little info about it from one of my favorite cookbooks, Chez Panisse Vegetables:

Sorrel should be pale to medium green and look very fresh. Keep sorrel in a plastic bag in the refrigerator, as you would other leafy greens, and use it within a day or so. To prepare sorrel for cooking, strip the leaves from the tough stems and rinds them in a large basin of cold water. Lift the leaves out of the water and drain thoroughly before cooking. When cooked, fresh green sorrel turns a distinctive olive drab. This is not unattractive, but some cooks add a little pureed spinach to dishes with pureed sorrel to brighten the color.

The English have added sorrel to spice up mixtures of salad greens since the early 18th century at least. The French prefer to use sorrel in soups and butter sauces for fish. The tart flavor of sorrel also has an affinity for potatoes.

Potato and Sorrel Gratin
About 24 large sorrel leaves
3 T unsalted butter
3 pounds russet or yukon gold potatoes
salt and pepper
nutmeg
2 cups milk, cream, or stock, or a combination
1/4 cup grated gruyere cheese
2 T grated reggiano parmesan cheese

Preheat the oven to 375 degrees. Butter a large gratin dish.

Wash, stem, and coarsely chop the sorrel, and cook until the leaves are completely wilted and have turned an olive green. Transfer to a plate.

Peel and slice thin the potatoes. Layer half the potatoes in the gratin dish, season well with salt and pepper, and grate a little nutmeg over them. Spread the cooked sorrel evenly over the potatoes. Make another layer with the rest of the potatoes and add enough milk to almost cover them. Season again with more salt, pepper,and nutmeg. Dot the surface with the remaining tablespoon of butter and sprinkle with the grated cheese. Bake about 1 hour, until the surface is nicely browned and the liquid is almost completely absorbed.

Serves 6-8.