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.
1/2cupchopped green onions,ramps or other wild onion
4-6cupsof chopped sorrel,packed
1quartchicken stock or vegetable stock
Melt 3 tablespoons butter in a soup pot over medium heat. Add the green onions or ramps and turn the heat to medium-low. Cover the pot and cook gently for 10 minutes.
While the onions are cooking, pour the stock into another pot and bring to a simmer.
Turn the heat up, add the sorrel leaves and a healthy pinch of salt to the pot with the onions and stir well. When the sorrel is mostly wilted, turn the heat back to medium-low, cover and cook 10 minutes. Stir occasionally. Mix in the flour and cook over medium heat for 3 minutes.
Whisk in the hot stock, stirring constantly. Bring this to a simmer.
To finish the soup, whisk together the egg yolks and cream. Temper the mixture by ladling a little soup into it with one hand, while you whisk the egg-cream mix with the other. Repeat this three times. (You are doing this to prevent the eggs from scrambling) Now start whisking the soup. Pour the hot egg-cream-soup mixture into the pot with the soup, whisking all the way. Add the final tablespoon of butter. Let this cook -- below a simmer -- for 5 minutes. Do not let it boil or the soup will break. Serve at once.
Serve this with bread and a nice white wine, or a floral beer like a Belgian.