From The Elements of Cooking:
photo by donna turner ruhlman
Custard: The most basic definition of custard is simply liquid thickened by eggs, a fundamental preparation, and idea, in both the savory and sweet kitchen. Custards give us some of our best dishes—quiches (above, photo by DTR), crème brûlée, and crème Anglaise (sometimes called custard sauce). Because they’re so prevalent in desserts, we tend to think of custards as sweet, though standard custard technique—liquid, eggs, seasoning, mixed and baked in a water bath—can be applied to virtually any flavor to which you want to bring the custard texture. An herb custard to garnish a savory soup, olive oil custard with a la grecque vegetables, tarragon or bone marrow with beef. Escoffier added diced custards (called a royale) to consommé. The liquid might be partly vegetable or fruit juice, though fat is a large part of the custard’s exquisite texture and shouldn’t be avoided altogether.
The key to a great custard is achieving an extraordinary texture. Returning to this definition, I’m surprised I didn’t mention the standard custard ratio, which is two eggs per cup of milk. The ratio is a baseline—it can be varied. One egg will thicken ¾s of a cup of liquid. One egg and two yolks will make a much richer custard than the standard ratio. Using a liquid with a lot of fat will set up a little thicker.
Custards are almost always cooked in a water bath, covered so that they don’t develop a tough skin on top. And cooking them for the right amount of time (knowing when to remove them from the oven, when they've still got some good jiggle so that carryover cooking finishes them, is critical).
One of my favorite custard preparations is the quiche. Notice how thick the one above is—you can’t make an excellent quiche in a pie dish. A quiche needs to be thick enough to achieve a gorgeous texture. Quiche is a savory custard and reminds us that while a custard is most often sweet, it shouldn’t be thought of as a sweet preparation but rather a neutral preparation that can be taken in any direction.
I love the custard, one of the great transformations of the egg.