This simple way of serving mackerel is very popular in France, and has existed for several centuries. It is mentioned in a cookery book of the beginning of the XVIII century, and was apparently a well-known dish even at that time.
The mackerel may be either boiled in salted water with a little fennel, or grilled and sprinkled with chopped fennel. There are several ways of preparing the gooseberry sauce, but in the average household it consists of a purée of gooseberries, made by putting the fruit in a saucepan with 1 or 2 tablespoons of water and sugar to taste, and simmering till the gooseberries are sufficiently tender to rub through a sieve. Sometimes the purée is added to a sauce of melted butter, or to a Béchamel sauce.