Add the vinegar, salt, pepper, Dijon, and garlic to a bowl. Whisk until the salt dissolves and the mustard is smooth.
Slowly pour in the olive oil while whisking constantly. Go slow at first. The dressing will turn thick, creamy, and pale as it emulsifies.
Dip a lettuce leaf and taste, then adjust the salt or vinegar to your liking. Toss with your salad right before serving. No whisk? Add everything to a jar, seal, and shake hard for 20 seconds.
Notes
Yield: Makes about 1 cup; serving size ~2 tablespoons.Storage: Keeps in a sealed jar in the fridge up to 2 weeks. The oil firms up cold, so bring to room temperature and shake before using.