You will need a fair bit of sorrel to make this recipe, as it cooks down into a puree alarmingly fast. You can buy sorrel at some fancy supermarkets, a lot of farmer's markets in the spring -- or you can garden your own or forage for it. (If you want to plant it in your garden, you can buy sorrel seed online.) If you can't find it, substitute watercress and use sour cream instead of regular cream.
Serve this with bread and a nice white wine, or a floral beer like a Belgian.