In a small bowl, whisk together vinegar, mustard, garlic, ¾ tsp. salt, and ¼ tsp. pepper. While whisking, slowly add olive oil until vinaigrette is emulsified.
When you're serving a salad for a dinner party, put vinaigrette in the bottom of a serving bowl and place greens on top. This can sit for an hour or 2. When ready to serve, toss greens and serve immediately.