Butter with Green Gooseberries

A compound butter from Ranhofer's line of hot and cold butters, meant as a finishing sauce for grilled oily fish rather than a spread. The whole thing hinges on the gooseberries being hard, green and sour.

• Use unripe fruitThese must be immature gooseberries, firm and acidic, picked well before they soften and sweeten. Ripe dessert gooseberries make the butter cloying and lose the sharpness that is the point of pairing it with mackerel.
• Quantities and yieldA pound of fruit to a pound of butter, roughly 450 g each, gives about two pounds of finished butter, far more than a domestic kitchen is likely to need. Halve or quarter it; the proportions scale straight down.
• Pounding, and the modern shortcutA food processor does in seconds what the mortar takes several minutes to do. Pulse the gooseberries first to a coarse pulp, then add cold butter cut in cubes and pulse again just until combined.
• Push it through a sieveGreen gooseberry skins are tough and the seeds are gritty. Work the pounded fruit through a drum sieve or fine strainer before it meets the butter, or the finished butter will be stringy on the plate.
• Keep everything coldGooseberries are largely water, and a one-to-one ratio is a lot of moisture for butter to carry. Work with the butter cool but pliable and stop as soon as it comes together; if it breaks, chill it and beat it back.
• Fine herbsFines herbes conventionally means parsley, chervil, chives and tarragon, chopped fine, not a dried mixed-herb jar. Tarragon in particular suits the fruit; use it with restraint.
• ServingRoll the finished butter in parchment into a log, chill until firm, and cut coins to melt over hot grilled mackerel, herring or sardines. It is not a cooked sauce and should not be heated on its own.
• KeepingThis is fresh fruit in butter with no acidification or cooking, so treat it as perishable: three or four days refrigerated, or wrap the log tightly and freeze for a couple of months.