Whisk canned soups, water, and onion soup mix together in a slow cooker until thoroughly combined.

Heat vegetable oil in a large skillet over medium heat. Add onion; cook and stir until browned, about 20 minutes. Transfer to a bowl, leaving oil in the skillet.

Brown cube steaks in the same skillet, about 5 minutes per side. Transfer to the slow cooker; top with onions. Cook on Low until cube steaks are tender, 5 to 6 hours.

Fill a large pot with lightly salted water and bring to a rapid boil. Cook egg noodles at a boil until tender yet firm to the bite, about 5 minutes. Drain. Serve cube steaks and mushroom gravy over egg noodles.