Add the French onion soup, cream of mushroom soup, gravy and water into the slow cooker and whisk until combined.
Add the cube steak and sliced onion into the slow cooker and toss with the gravy mixture.
Cook on low for 6-8 hours.
Mix the cornstarch and 3 tablespoons of water together in a small bowl. And add to the gravy mixture and toss until coated. Cook on high another 30 minutes to thicken the sauce.
Serve over mashed potatoes, pasta, or rice.
Notes
Store in an airtight container in the refrigerator for up to 3 days or freeze for up to 2 months.
Cream of mushroom is classic, but you can swap in cream of chicken or cream of celery for a slightly different flavor.
Cook on low for 6–8 hours for the most tender texture. Cooking on high may cause the meat to toughen.