Preheat oven to 350°F. Melt butter in a large frying pan. Add onion and garlic; simmer until onions are transparent, about 5 minutes. Set aside to cool. In a small mixing bowl combine barbecue sauce and mustard. Add half sauce mixture to onions.
Put crackers in a zip lock bag and crush with a rolling pin until coarse. In a small bowl cover cracker crumbs with buttermilk and let soak 5 minutes; mix in eggs and mash with a fork to oatmeal consistency. In a large mixing bowl combine beef thoroughly by hand with onion mixture, cracker mixture, parsley and pepper. Press into a 9 x 5 loaf pan and place on a sheet tray.
Bake for 1 hour or until internal temperature reaches 160°F degrees. Place loaf pan on a cooling rack and let rest 10-15 minutes. Transfer to a cutting board or platter, top with remaining sauce, slice and serve.