To make the finished vinaigrette, it depends on what he wants (his words, not mine...this is like hitting a moving target). SOoooo, ideally you don't make a batch ahead of time but rather mix it up in the bottom of the salad bowl that you are using
For salad greens serving 8-10 people, begin with 3-4 TBS base, add 3-4 TBS marinated onions, a splash of good red wine vinegar (see MJ dishes it out below) and minced parsley. Taste for acidity and seasoning. Add more salt, pepper, red wine vinegar or olive oil if needed. You need to find YOUR happy place.
Xavier has MANY variations. He might add minced garlic, or chives, or basil, or pecans. or a combo of these. I have seen him add a chopped hard boiled egg. Possibilities are endless. The base can also be used as is. Once you start using it, you will get the hang of it and have your own variations.