This, the most elaborate of the classic compound butters, is well worth the effort. It is excellent served on hot or cold fish and meats. It has a softer texture than most compound butters because of the olive oil it contains.
Crush 1 small peeled garlic clove in a mortar and work to a paste. Add 4 anchovy fillets, 1 chopped chornichon (sour gherkin), and 1 tablespoon (15 milliliters) capers and work again to a paste. Blanch the leaves from ½ small bunch each parsley, watercress, chervil, chives (chopped before blanching); 1 sprig tarragon; 1 small handful spinach leaves; and 2 chopped shallots in boiling salted water for 30 seconds. Drain, rinse with cold water, and wring dry in a kitchen towel. Combine with the garlic mixture and grind to a paste in the mortar or in a food processor. Add 1 raw egg yolk and 2 cooked egg yolks and work again until smooth. Add 6 tablespoons (90 grams) butter and work through a drum sieve. Gently work ¼ cup (65 milliliters) extra-virgin olive oil into the butter with a wooden spoon (remember, whisking turns olive oil bitter). Season with salt, pepper, cayenne, and a little lemon juice or good wine vinegar.