A quiche is a savory custard — but it’s not the only one. There are flans, sformato, and gnaffron (and maybe a few I am forgetting about).
The differences between savory flan, savory custard, Italian sformato, and French gnafron are minimal, if they exist at all which is why they are usually grouped together and simply called savory flan in American cookbooks. (If they are included at all. Cookbooks mostly just toss in a few quiche recipes which have crusts).
The whole subject is a mess of different names and minor differences but the same ingredients. There are often differences in the size of the dish it is baked in, and what kind of dish — ramekin, custard cup, casserole.
And, then there is the subject of “texture”.
Most of the various savory custard dishes are cooked in a water bath (bain-marie). That’s when you cook the custard containing ceramic or glass dish (custard cup, ramekin, etc.) in a bigger dish/pan filled with water, and the whole lot is baked in the oven. A bain-marie evens out the temperature, for a smoother, creamier mouth-texture. (The longer and slower an egg cooks, the more delicate the final product).
I always put the larger dish/pan into the oven, add the small cups, and then pour already warmed water into the larger dish. Some cooks recommend cold water, some boiling. I met it in the middle — warm. (Although, after lots of trials, I never found any real difference between the starting water temperature and the final texture.)
The cooking science with a bain-marie,
The proteins in eggs just begin to coagulate at about 150 degrees Fahrenheit, egg whites at about 140 degrees. If you surround your custard with a pan of water the water will insulate it from direct heat, and provide a constant temperature that will cook the egg proteins slower than if they were just in the oven at 325 degrees.
Think of it this way — if the oven is set to 325 degrees Fahrenheit, the water filled pan will never go higher than 212 degrees Fahrenheit.
Crusted quiches never use a bain-marie because the crust would not properly crisp, and the crust itself has some insulating qualities for the egg. Plus, the texture desired with a quiche is more desirable as a slightly rougher, tougher egg dish. So, when recipes dub something a “crustless quiche” they’re lying to you. it’s a custard, or a custard by another name.
1½ pounds walnuts
2 cups minced onions
4 tablespoons unsalted butter
¼ cup dry white wine
¾ cup heavy cream
6 eggs, whisked
pepper, to taste
1 teaspoon freshly squeezed lemon juice
¼ teaspoon sugar
Preheat oven to 350 degrees.
Cook walnuts in boiling salted water for 5 minutes. Set aside, and let cool. Chop into ¼-inch pieces. Sauté onions in butter, and add chopped walnuts. Cook several minutes until onions are lightly browned. Pour in wine, and cook to burn off alcohol (several minutes). Add cream, and cover and cook 10 minutes on very low heat, occasionally removing cover to stir. Remove from heat, and let cool.
Grease six to eight ramekins or custard cups. Fold eggs into cooled walnut mixture. Mix well. Pour into custard cups until three-quarters full. Place cups in deep baking dish, and pour in water to about halfway up sides of cups. Place in oven, and bake 35 to 40 minutes or until knife inserted comes out clean.