Blend all ingredients in a food processor until smooth. Taste and adjust salt and pepper as desired, and use extra milk to thin the dressing if you need to. It should be the consistency of heavy cream.
Set aside while you prepare your salad, or store, sealed, in the fridge for up to a week if you’re prepping ahead.
Notes
This dressing is actually better made ahead of time! Even if you can just let it sit for 20 minutes before dressing your salad it will make a difference. It keeps up to a week, covered, in the fridge.
You can directly substitute thick Greek style yoghurt for the mayonnaise if you'd like a lighter dressing. The taste will be slightly different but still delicious and with similar 'body'.