Mayonnaise is one of the few miracles of French cuisine that has become common in modern-day America. Sadly, we mostly eat mass-produced mayo from a jar, which -- though it's far from the worst prepared food in the world -- tastes nowhere near as good as homemade. You might think making mayonnaise from scratch is too challenging a task, but the truth is that you can make real mayonnaise in about five minutes with a few ingredients you undoubtedly already have.
Making real mayonnaise at home requires either upper-body effort or a food processor or blender. That's because making an emulsion (in this case, a creamy, cohesive mix of oil, acid, and egg) requires constant, vigorous stirring. If you stop mixing for just a few seconds, your mayonnaise will break (separate into its component parts) or end up too runny. I generally let a machine do the work for me, but if you don't have a blender or food processor, a bowl, a whisk (or even a fork), and some tenacity will do the trick.
Lighter Shade of Pale
If you care about the color -- yolk-only produces a pretty yellow mayo, while whole egg is much paler -- separate an egg before you begin: Crack the shell in two over a bowl and pass the egg repeatedly from one half of the shell to the other; the white will drop down into the bowl, while the yolk will stay in the shell. (You can cover and refrigerate the egg white and use it in another recipe, like angel food cake, meringues, or seven-minute frosting.) Transfer the yolk to another bowl (or your blender or food processor) and incorporate a couple of teaspoons of Dijon mustard.
Come Together Now
See the recipe for blender/food processor instructions, which are really the easy, foolproof way to go, and make it easy to add many other flavors. If you're making mayo by hand, the next part is what determines whether your mayonnaise will come together or break; it takes a little patience and precision. Start beating in oil (grapeseed and corn are good; use olive oil only if you want its distinct flavor) a little at time-start out with literally just a drop. Keep whisking vigorously as you dribble in the oil very slowly. (A squeeze bottle is handy here, though a measuring cup with a spout works also.) After you've added maybe a quarter of the oil, the mixture will have become thick and creamy, which means that it's emulsified. After this point you can add the rest of the oil a little more quickly -- but it's better to err on the side of adding the oil too slowly than to risk ruining the mayonnaise by drizzling it in too fast.
If your mayonnaise breaks or never emulsifies in the first place, throw it out and start again, working a little more slowly this time -- eventually you'll get the hang of it. Remember, it's only oil and an egg -- the ingredients for a batch of mayonnaise usually cost under a dollar, so you can afford a few practice runs or an occasional encounter with bad luck.
If you're against homemade mayo for health or ethical reasons (you might be vegan, for example), you can still make a terrific approximation of the real thing with silken tofu. Don't scoff -- tofu mayonnaise is still better than most of the so-called "real" mayonnaise you buy at the grocery store. I love mixing it with sweet white miso to create a sweet-and-savory, rich-but-not-too-rich dip for vegetables and spread for sandwiches, but if you want something more akin to traditional mayonnaise, just leave out the miso.