Preheat barbecue to medium. Brush peaches with melted butter. Grill core side down for 2-3 minutes. Remove from heat and let cool. Slice peaches. Place in airtight container. Place arugula and sautéed pancetta in separate containers in a picnic basket, along with grilled peaches, fresh Mozzarella and lemon vinaigrette. Encourage guests to build salad outside.