In a 2-cup glass measuring cup, whisk together the vinegar, Dijon mustard, whole grain mustard, garlic, egg yolk, salt, and pepper. Slowly whisk in the olive oil to make an emulsion. You should have about 2/3 cup vinaigrette.
When ready to serve, place the salad greens in a bowl and pour on enough vinaigrette to moisten. Serve immediately.