Combine the vinegar, mustard, and garlic (if using) in a small bowl and whisk vigorously until completely smooth. The mustard acts as an emulsifier, so beat it well to activate its binding power.
Begin adding the olive oil in the thinnest stream possible while whisking constantly — we're talking droplets at first, then a pencil-thin drizzle. Watch the mixture transform from separate liquids into a unified, glossy emulsion that coats your whisk.
Taste and adjust with pinches of salt and fresh pepper until the flavors sing in harmony — you want bright acidity balanced by the oil's richness. The vinaigrette should taste bold enough to enhance, not overpower, your greens.
Dress your salad immediately for the best texture, or transfer to a jar and refrigerate for up to a week. If it separates during storage, just shake vigorously or re-whisk before using — the emulsion will come right back together.