Place the onions in a jar; fill the jar about a third of the way with vinegar, then fill to the top of the onions with water.
Add salt and sugar, shake, and keep the onions in the fridge for a few hours or a few days.
Shake all dressing ingredients in a jar.
Prep all your salad greens by chopping the romaine and herbs and gently tearing the lettuce leaves into bite sized pieces.
Just before serving, toss greens, herbs, pickled onions, and dressing together, starting with about half of the dressing and adding more as necessary.
Taste and adjust, then add salt and pepper to taste.