To prepare the dressing, use a whisk to combine the oil, vinegar, mustard, honey, and salt.
To assemble the salad, fill a large bowl with the mixed greens, then top them with the apple, onion, walnuts, and feta, if using.
Just before serving, generously drizzle some of the dressing over the salad and toss well so that everything gets evenly coated. You can save any leftover dressing in an airtight container in the fridge for up to a week.
I like to use a balsamic vinegar that has been aged for at least 3 years for best flavor. (It's a little more expensive than the cheapest balsamic options at the grocery store.) If you use one that hasn't been aged as long, you might want to add a little more sweetener to balance out the acidity.