In a small jar or bowl, combine the olive oil, balsamic vinegar, honey, Dijon mustard, salt, and pepper. Shake or whisk until well blended to make the dressing.
In a large salad bowl, combine the baby spinach, strawberries, blueberries, blackberries, pecans, feta cheese, and red onion.
Drizzle the dressing over the salad just before serving and toss gently to coat.
Serve immediately and enjoy.
Notes
For best results, add the dressing right before serving to keep the spinach crisp.