To make sauce: In a saucepan, sauté onion and garlic in oil until softened and translucent. Once translucent, add dash of salt and cider vinegar and let sauce evaporate a bit. Add brown sugar, honey and bourbon and let reduce down. Add peaches from grill and stir in remaining ingredients. Simmer, uncovered, stirring occasionally for 45 minutes or until peaches are very tender. Remove sauce from stove and purée in a blender (use caution when blending hot liquids).
Let the sauce cool down and then put in refrigerator until needed.
Grilling chicken: Prepare a grill to medium-high heat. Brush grill rack with oil. Salt and pepper chicken, then grill chicken until browned and almost cooked through, 4 to 5 minutes per side. Brush on all sides with reserved sauce; grill until glazed and cooked through, 1 to 2 minutes per side, or until internal temperature reaches 165 degrees. Slice crosswise.
To serve: Toss spinach with dressing and garnish with walnuts and extra grilled peach slices. Serve peach barbecue chicken on top and enjoy.
Serves 6