Preheat your grill or grill pan to medium/high. Brush the watermelon slices with olive oil. Grill on the first side for 4-5 minutes or until caramelized and grill marks form. Flip the watermelon and top with slices of mozzarella. Cover grill pan with aluminum foil if using, or close the lid of the grill and cook for another 2 to 3 minutes until the cheese is melty. Remove from grill, sprinkle with pesto, extra pistachios, and a drizzle of honey.