Thinly slice one medium-sized onion and place the slices in a jar. Pour in enough vinegar to fully submerge the onions, allowing them to soak for at least 30 minutes.

In a small bowl, combine 1/4 cup of canola oil, 2 tablespoons of vinegar, and 1 teaspoon of mustard. Whisk together until emulsified and smooth.

Wash the mixed tender greens thoroughly and dry them using a salad spinner. Transfer to a large mixing bowl.

Add the pickled onions and chopped fresh herbs to the bowl with the greens. Toss gently to mix.

Drizzle the prepared vinaigrette over the salad mixture and toss gently using tongs to coat evenly.