Spread flour into a shallow dish. Dredge ribs in flour to coat.
Heat vegetable oil in a skillet over medium heat; cook ribs in hot oil until completely browned, 2 to 3 minutes per side. Remove ribs to a casserole dish.
Melt butter in the hot skillet. Cook and stir onion in melted butter until translucent, about 5 minutes. Stir ketchup, vinegar, Worcestershire sauce, brown sugar, mustard, and salt with the onion until you have a smooth sauce. Pour water into the sauce; cook and stir until hot. Pour sauce over the ribs. Cover dish with aluminum foil.
Cook in preheated oven, spooning sauce over meat several times during baking, for 90 minutes. Turn ribs and continue cooking and basting with sauce until the meat is completely tender, about 90 minutes more.