In a small bowl, combine the balsamic vinegar, olive oil, poppy seeds, honey, Dijon mustard, garlic powder, salt and pepper.
Whisk thoroughly for 2-3 minutes until the dressing is well combined. Alternatively, place all the dressing ingredients in a jar with a lid and shake until combined.
Place the spinach in a large serving bowl or divide it among individual serving bowls.
Top the spinach with sliced strawberries and sliced red onion.
Drizzle the dressing over the salad and gently toss until all the ingredients are lightly coated.