This rich condiment graces chef Jeremiah Tower's Peppered Fillet of Beef.
Prepare an ice-water bath. Bring a large saucepan of water to a boil; add salt. Combine spinach, shallots, watercress, parsley, chervil, chives, and tarragon in a fine sieve. Carefully place sieve in boiling water until greens are wilted, about 30 seconds. Remove, and place in ice-water bath to cool. Remove, and squeeze dry. Transfer to a food processor fitted with the metal blade.
Add cornichons, anchovies, capers, garlic, and cayenne pepper. Season with salt and black pepper. Process to a smooth paste. Add cooked and raw yolks, and butter. Process until thoroughly combined. Transfer to a medium bowl, cover, and refrigerate for 2 hours.
Just before serving, remove from refrigerator, and slowly whisk in olive oil. Add vinegar, and whisk to combine. Adjust seasoning, if necessary.
Please note that raw eggs should not be used in food prepared for pregnant women, babies, young children, the elderly, or anyone whose health is compromised.