In a small bowl or jar, whisk together balsamic vinegar, Dijon mustard, honey, salt, and pepper.,Slowly drizzle in olive oil while whisking until the dressing is emulsified and smooth.,In a large salad bowl, combine spinach, sliced apple, cranberries, and cashews.,Add optional red onion and cheese, if using.,Drizzle the vinaigrette over the salad just before serving and toss gently to combine.,Serve immediately for best texture and flavor.