The 3:1 Vinaigrette Rule for Classic French Dressing

A classic French vinaigrette is 3 parts oil to 1 part vinegar, emulsified with about half a teaspoon of Dijon mustard. For four portions, that's 3 tablespoons of extra-virgin olive oil, 1 tablespoon of red wine vinegar, the mustard and a pinch of salt, with the oil whisked in slowly so the dressing holds instead of separating.

You shake it, pour it, and by the time the bowl reaches the table there's a slick of oil on the leaves and a puddle of vinegar at the bottom. That's not a ratio problem. It's an emulsion problem.

The one change that fixes it

Vinegar is mostly water, and oil won't stay mixed with water on its own. Most home vinaigrettes fail because the oil goes in all at once and nothing is there to hold it.

Mustard first, oil last, and slowly.

Dijon isn't only there for flavor. Mustard contains mucilage, which coats the tiny oil droplets and thickens the vinegar around them so they can't find each other and pool back together. Even half a teaspoon makes a measurable difference in how long the dressing holds, and at that amount it won't take over the taste. If you don't love mustard, use it anyway.

The slow stream matters just as much. Each addition of oil has to be broken into droplets and coated before the next one arrives. Dump it in and you'll get a broken, greasy dressing no matter how hard you whisk afterward.

How to make it

1. Steady the bowl. Use a small, flat-bottomed bowl, or set a rounder one on a damp towel, so you can whisk with one hand and pour with the other.
2. Macerate the aromatics. Put 1 teaspoon of very finely minced shallot (or a crushed garlic clove) in the bowl with a small pinch of salt and let it sit for 60 seconds. The salt softens the raw bite and pulls out juice that blends into the dressing. Rough chunks of shallot taste harsh, so mince until it nearly disappears.
3. Add the mustard and vinegar. Whisk in half a teaspoon of Dijon and 1 tablespoon of vinegar. Season now, because salt dissolves in the vinegar but won't distribute evenly once the oil is in.
4. Stream in the oil. Pour 3 tablespoons of room-temperature olive oil in a thin, steady stream while whisking constantly. Cold oil straight from the fridge pours unevenly and emulsifies poorly, so give it ten minutes on the counter first.
5. Taste it on a leaf. Dip a piece of whatever you're dressing. Flat means more acid, sharp means more oil, and if the flavors seem separate, add a pinch more salt. A tiny pinch of sugar will round off harsh acidity without making it taste sweet.
6. Dress at the last moment. Toss right before serving. Even a stable vinaigrette will wilt delicate greens within a few minutes.

In a restaurant, vinaigrette gets made by the quart before service and lives in a squeeze bottle that's shaken before every plate. At home, a jar shake is fine if the mustard is in there, but it won't hold as long as a whisked one. For a single salad tonight, shake it. For dressing that has to sit on the table through a meal, whisk.

Related: Salad Dressing Won't Emulsify? Here's Why

Adjusting the ratio and the vinegar

Three to one is a starting point, not a law. A sharp sherry vinegar needs more oil to balance it than a mild Champagne vinegar does, and a hearty grain salad can take more acid than butter lettuce can. Cold food mutes flavor, so at room temperature the dressing should taste a little stronger than you think it needs to.

Red wine vinegar is the traditional choice and works on most greens. Champagne vinegar is lighter and suits delicate lettuces. Sherry vinegar has a nutty flavor that stands up to strong cheeses and bitter leaves like radicchio. I'd skip distilled white vinegar here. It's too sharp and one-note for a finished dressing. More on balancing acid applies directly.

A vinaigrette has nowhere to hide, so the oil sets the ceiling. Use a mid-weight extra-virgin olive oil. Very grassy, peppery oils can overpower a plain green salad, so save them for something robust like radicchio or roasted peppers. Once the base is reliable, swap half the olive oil for walnut oil or stir in fresh herbs.

How long does it keep?

A plain base of oil, vinegar, mustard and salt keeps up to two weeks in a sealed jar in the fridge. Olive oil firms up when cold, so let the jar sit at room temperature for five minutes and shake it before using.

Fresh garlic, shallot or herbs change that. The National Center for Home Food Preservation advises refrigerating homemade garlic-in-oil and using it within 4 days, and I'd treat an aromatic vinaigrette with the same caution. If you want a two-week batch, store the base plain and add shallot to each portion as you use it.

Related: The 5 French Mother Sauces and What Each One Teaches