Preheat oven to 325°.
Heat oil in a nonstick skillet over medium-high heat. Add onion to pan and cook 4-5 minutes or until onion is tender.
In a large bowl, combine the onion, Buffalo wing sauce, shredded beef, cream cheese, and 1 cup cheddar cheese. Stir well until evenly combined.
Place beef mixture to an 8x8-inch baking dish and bake at 325° for 15-18 minutes. Remove from oven and top evenly with remaining 1 cup cheddar cheese; turn broiler on high. Broil for 2 minutes or until cheese is melted. Top evenly with parsley and serve immediately with celery sticks and tortilla chips.