Preheat oven to 300º. Make 1/4-inch-deep cuts in fattiest side of pork. Rub pork with kosher salt and freshly ground pepper. Arrange onion and next 3 ingredients in a large roasting pan; drizzle with oil, stirring to coat. Place pork, fattiest side up, on vegetables in pan.
Bake at 300º for 3 1/2 to 4 hours or until tender. Let stand 10 minutes. Remove pork from pan, reserving 1 Tbsp. drippings. Discard remaining drippings and vegetables.
Cook pork in a 12-inch cast-iron skillet over medium heat 3 to 5 minutes on each side or until browned and crisp.
Stir together peaches and next 3 ingredients. Heat reserved 1 Tbsp. drippings in a large skillet over medium heat. Cook peach mixture in hot drippings, stirring often, 3 to 5 minutes or until thoroughly heated. Season with additional salt and pepper to taste. Slice pork, and serve with warm peaches and arugula.
Pantry Key. Pork Belly: This new chef fave has nothing to do with the digestive tract-it's uncured bacon. Order it from your butcher or .