In a large salad bowl, combine the fresh spinach leaves, sliced mushrooms, halved cherry tomatoes, thinly sliced red onion, and any other optional add-ins you'd like to include.
In a small bowl or jar, whisk together the extra virgin olive oil, balsamic vinegar, Dijon mustard, honey or maple syrup, salt, and pepper until well combined.
Drizzle the dressing over the salad ingredients in the bowl, starting with a small amount and adding more as needed, tossing gently to coat the salad evenly.
Sprinkle crumbled feta cheese or goat cheese (if using) and sliced almonds or chopped walnuts over the top of the salad.
Serve immediately as a side dish or add grilled chicken, shrimp, or tofu to make it a complete meal.
Enjoy your fresh and flavorful Spinach Salad!