1. Spray the watermelon with cooking spray, and grill on both sides. Lay on serving plates.
2. In a large bowl, mix salad greens, endives, red onion, basil, and avocado.
3. In a medium bowl, whisk shallot and mustard. Add vinegar, lime juice, maple syrup, oil, salt and pepper, and mix.
4. Toss and serve over the watermelon.