That’s the little bunch of round, soft, green leaves that taste like lemon, sort of.
Sorrel is a relative of rhubarb, and it has a sharp, sour taste. The French love it, and in England they used it in soups, sauces, and custards for centuries. It is generally used as a flavoring puree—you just simmer the cleaned leaves in a tiny amount of water, or stir them into a pan of butter. It makes a tart sauce which is delicious with root vegetables or fish or chicken.
To prepare: clean sorrel by floating it in water, then drain it. If you’re going to cook it, you’ll need to strip the leaves from the stems. It’s not hard. You can add it to salad, raw, or include it in soups and casseroles.