Place flour in a large resealable plastic bag. Add steaks, a few at a time, and shake until completely coated. In a skillet, cook steaks in oil until lightly browned on each side. Transfer
to a slow cooker. Add the onion and 2 cups water. Cover and cook on low for 8 hours or until meat is tender. In a bowl, whisk together gravy mixes with remaining water and mushrooms. Add to slow cooker; cook on high 1 hour longer. Serve over mashed potatoes.