Pour the balsamic vinegar into a small saucepan and bring to a boil. Reduce heat and simmer for 15-20 minutes until the balsamic vinegar is significantly reduced and starting to become syrupy.
Meanwhile, turn a grill or nonstick skillet to high heat. Sprinkle the chicken with the seasoning on both sides and drizzle with the olive oil. Transfer to the hot grill and cook until no longer pink on the inside and nicely browned on the outside. Let rest for 5 minutes; cut into small cubes.
Layer the spinach with the chicken, watermelon, blue cheese, almonds, and drizzle with the balsamic reduction to taste. Top with additional olive oil if desired