In a small bowl, whisk together olive oil, balsamic vinegar, honey, salt, and pepper to make the dressing.
In a large salad bowl, combine spinach, mango slices, blueberries, red onion, and toasted walnuts.
Drizzle the dressing over the salad and toss gently to coat.
Sprinkle crumbled feta cheese on top just before serving.
Serve immediately for the freshest flavor.