Place vinegar and sugar in a large mixing bowl and whisk to blend. Add sliced strawberries and turn to coat. Set aside to marinate for 20-30 minutes, turning occasionally.
Drain and transfer strawberries to separate bowl, reserving balsamic juices; set aside. Slowly drizzle olive oil into balsamic mixture, whisking continuously until emulsified. Season to taste with salt and pepper.
Place spinach in a large bowl and add half of dressing, tossing until leaves are coated and dressing is distributed. Divide spinach amongst individual salad plates. Place 3-4 slices of avocado decoratively over spinach, alternating with balsamic strawberries. Repeat with each salad plate. Drizzle a little more dressing over each salad, and garnish with a sprinkling of sesame seeds. Serve and enjoy.