Ahh, super easy and super French. This is my moms recipe, so thank Alison if you like it.
In a small bowl, combine the mustard and vinegar. Add the olive oil in a slow drizzle, whisking with a tiny whisk or a fork. It should emulsify beautifully. (Thank you, mustard!) Season with pepper.
I don't know my exact measurements, so I've approximated, but you'll have to adjust to taste. If the mustard is a very prominent taste, adjust your oil and vinegar as you see fit. This is great on any classic salad.