Cut four, 1-inch thick, half-moon slices of watermelon. Reserve any remaining watermelon for another use. Mix vinegar, oil, lemon juice, rosemary, salt and pepper in a small bowl. Reserve 2 tablespoons for drizzling over grilled watermelon. Place watermelon steaks in a glass dish, add remaining marinade and refrigerate for 20 minutes, turning the watermelon halfway through marinating time.
Meanwhile, for the Walnut Gremolata, mix walnuts, parsley and lemon peel in small bowl. Set aside. Remove watermelon steaks from the marinade. Reserve leftover marinade for brushing watermelon during grilling.
Grill watermelon steaks over high heat for 2 to 4 minutes per side or until grill marks appear, brushing with leftover marinade continuously.
To serve, cut each watermelon steak in half, drizzle with reserved marinade and sprinkle with Walnut Gremolata.
Welcome to the grill, watermelon. Marinate these thick-cut “steak” fruit slices in a mixture of white balsamic vinegar, lemon juice and rosemary. Grill watermelon slices and top them with a gremolata of walnuts, lemon zest and fresh parsley for knock-the-grill-tongs-out-of-your-hand taste.