Soak the onion slices in a bowl of cold water to remove their acidic bite. Set aside.
Whisk together all of the dressing ingredients.
Place all of the salad ingredients into a large bowl and season with a little salt and lots of cracked black pepper. Drain and pat dry the onion slices on a piece of paper towel before adding them to the bowl.
Add the dressing and gently fold the salad together, taking care not to break up the feta too much. Add more salt and pepper if needed before serving.
