A simple, lightning quick sauce can transform plain white fish such as cod, scrod (baby cod) or hake into something delicious, and a marvelous silky filet like Chilean sea bass into plain delectable.

The defining ingredient is Pommery mustard, crafted originally at a monastery in Meaux, and served to the kings of France since 1632. It still arrives in a little beige stoneware crock with old fashioned label (although the traditional cork closure covered with red sealing wax fell by the wayside a decade or so ago), brightens the refrigerator shelf, and beckons to improve every sandwich. Its texture and taste remain memorably distinctive—mustard seeds that are plump, whole and succulent, perfumed with fine vinegar and spices, and just enough tang.

For the present dish, select a filet that is more or less uniformly thick in order that it cook evenly. I usually ask the fish monger to favor the tail end of the fish, approximately half a pound per person for good eaters. Be aware that Chilean sea bass, a rich deep water creature, requires more cooking time than other white fish; filets thicker than an inch dry out and toughen before their center cooks through and flakes. If you must work with thick pieces, slice them horizontally.

In a little bowl, blend 2 or 3 tablespoons of olive oil, juice of half a lemon, and a half teaspoon or more of Pommery. Adjust proportions to suit your taste. Arrange the fish in a low broiling pan, smearing a bit of oil beneath the skin side so the fish doesn’t stick. Pour the flavored mixture evenly over the top surface. Broil without turning just until the fish flakes