In a small bowl, whisk together the EVOO, apple cider vinegar, honey or maple syrup, Dijon mustard, salt, and pepper until well combined. Taste and adjust the seasoning if needed.
In a large mixing bowl, combine the baby spinach, apple slices, red onion, and dried cranberries.
Drizzle the dressing over the salad and toss gently to coat all the ingredients well.
Just before serving, add the toasted walnuts and feta cheese (if using) to the salad and give it a final gentle toss.
Serve the salad immediately as a refreshing side dish or a light main course.