To make the balsamic vinaigrette, or in a jar with a tight-fitting lid or a bowl, combine olive oil, balsamic vinegar, Dijon mustard, and honey. Whisk or shake to combine, then season to taste with salt and pepper.
In a large bowl, combine lettuce, apples, red onion and walnuts. Drizzle with the vinaigrette and toss to combine. Sprinkle with feta and serve immediately.
Honey: Or substitute an equal amount of pure maple syrup.
Walnuts: Toast walnuts in a medium skillet over medium heat, heat walnuts until browned and fragrant, stirring occasionally, about 2 to 5 minutes.
Yield: This recipe makes at least 8 cups of salad, enough for 4 generous servings, 2 cups each.
Storage: Store leftovers covered in the refrigerator for up to 4 days (for best results, store extra salad undressed).