This dressing is traditionally whisked together in the bottom of a large salad bowl. After it's made, lettuce and any other ingredients are added to the bowl and gently tossed to completely coat the greens, while leaving any excess dressing at the bottom of the bowl. It's a fabulous technique that leads to perfectly dressing salads, but whipping up a double, triple, or quadruple batch in a small jar to use over the course of a few days is a nice option, too.
If using immediately, whisk together the oil, vinegar, shallot if using, and mustard in the bottom of a large salad bowl. Add salt and pepper to taste.
Add whatever salad you're making and toss to combine—as you toss the salad will pick up as much dressing as it requires to be coated (for more heavily coated salad, keep tossing), and excess dressing will naturally stay at the bottom of the bowl. It's a far better method that dumping heavy liquid dressing onto delicate greens.
Note: The best way to taste a dressing is to dip a lettuce leaf (or whatever you're going to dress with the dressing) in and take a bite—that way you're tasting the dressing as it will be eaten.
To make ahead, or for smaller salads, either whisk ingredients in a small bowl or measuring cup or put ingredients in a jar, cover, and shake to combine. Keep, covered and chilled, up to 1 week. Note and be warned that the olive oil will congeal in the cold and look pretty gross. While it looks nasty, rest assured that nothing horrible has happened to the dressing; it will look completely normal and taste completely delicious once it comes back to room temperature and the oil has "melted" back to liquid. Hasten that process along, if you like, by whisking or shaking it frequently to encourage it.