This is a classic French bistro-style dressing, in its use of wine vinegar and Dijon mustard, both of which you must really stock in your larder. Don’t be alarmed by the raw shallot/onion in the dressing. It’s not noticeable, but provides a mellow sweetness in the dressing. You can substitute the shallot/onion for a clove of garlic, for an equally delicious, but slightly sharper dressing.
You can make more than you need, and store the rest. Personally, I prefer to make it fresh as it literally takes minutes to put together. You can even make the dressing on the base of the serving bowl. Saves washing up, and you can pile on the leaves atop the dressing, and leave in the fridge. Toss everything together when ready to eat.