Sprinkle both sides of the steaks lightly with salt and pepper, pressing in gently so pepper adheres to the meat. Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Pan fry steaks about 2 minutes per side until meat is medium rare. Remove from pan and place on a platter. Cover to keep warm. Add remaining oil to the pan along with the onions. Cover and cook over medium heat, stirring often, for 5 minutes or until onions are softened. Uncover and raise heat to medium-high. Sauté, stirring frequently, for about 2 minutes or until onions are lightly browned. Add mustard, wine and water to pan and simmer for about 45 seconds, stirring up browned bits clinging to bottom of pan. Add any meat juices that have accumulated from the cube steaks to the sauce. Taste and add salt and pepper if needed. (If steaks have cooled, return to the pan and cover for 1-2 minutes to re-warm.) Serve steaks with onions spooned over or turn them into sandwiches using kaiser rolls.