Divide ground beef into 12 flat patties. Partially saute bacon - the fat should start rendering, but it shouldn't be crispy. Drain bacon. Mix pecans, parsley, onion, and butter together. Spread mixture on 6 of the patties. Top with remaining 6 patties and seal the edges with the partially sauteed bacon and secure with a wooden toothpick. Broil or grill 10 to 15 minutes, or until cooked to desired doneness, turning once.