This is really really good! It’s a thick dressing; you may want to add more oil or even some water to thin it, though that will also dilute the flavor of the onion some. There is sugar here; it balances the acidity of the onion. If you were to use sweet onion, no sugar would likely be needed. Using red onion would probably require less sugar.

1/3 c apple cider vinegar, white wine vinegar, or mild red wine vinegar
1 t dry mustard
1 t celery seeds or caraway seeds
1 T sugar or honey or maple syrup or agave
salt and pepper, to taste
1 medium yellow onion, chopped
1 c preferred salad oil, plus extra if preferred

1. In a blender, combine the vinegar, mustard, celery seeds, sweetener, salt, and pepper. Whizz. Add the onion and whizz again. Then add the oil in a stream until well emulsified. Or, just throw everything into the blender and whizz until emulsified.  (I think blending in stages really works best for making mayonnaise.) If the dressing is too thick, add some more oil or water until you get a consistency you like.