In a small saucepan, over medium heat, add the balsamic vinegar and crushed garlic and bring to a simmer. Let simmer for 5-10 minutes, until the sauce thickens. The texture should be syrup-like.
In a large bowl, combine the arugula, watermelon, red onion, feta, mint, pistachios, and olives. Drizzle with the reduced balsamic vinaigrette and toss gently. Enjoy!