This greatest of all butters is traditionally made in a mortar with a pestle, but a food processor will do. It is very good with cold poached fish, especially salmon, but is equally delicious with hot grilled fish. Spooned between slices of cold roast veal or pork, the slices reassembled, left for a day, and served at room temperature, it creates a lifelong memory.
Blanch the spinach, watercress, herbs, and shallots in boiling water for 1 minute. Drain, refresh under cold water, and squeeze dry. Put them in a mortar or food processor. Add the cornichons, anchovies, capers, garlic, salt, pepper, and cayenne. Work with a pestle or process to a smooth paste. Add the egg yolks and the butter and process again until thoroughly mixed. Put through a food mill, fitted with the fine-mesh disk, into a bowl or back into the mortar and whisk in the oil by hand. The mixture should be glossy and as smooth as velvet. Beat in the vinegar and taste to see if it needs more salt and pepper.
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