Onion sauce: Heat oil in a skillet over low heat. Add in onion and cook for a few minutes until softened. Stir in water, ketchup, and salt. Cover pan and cook for about 20 minutes, stirring occasionally, until onions are completely soft and saucy.
Boil hot dogs until cooked. Place in buns and then layer sauerkraut, onion sauce, and French’s spicy brown mustard on top.