Place the beef tips in the bottom of your slow cooker in an even layer.
Add the sliced mushrooms on top of the beef.
Sprinkle the onion soup mix evenly over the beef and mushrooms.
Spoon the cream of mushroom soup over the top. Do not stir.
Place the lid on the slow cooker. Cook on low for 7 to 8 hours or on high for 3 to 4 hours, until the beef is fork-tender.
Stir everything together gently to combine the gravy with the beef and mushrooms.
Serve over mashed potatoes, egg noodles, or rice.
Notes
Do not add water or broth. The cream of mushroom soup provides all the liquid this recipe needs.
Browning the beef in a skillet before adding it to the slow cooker is optional but adds deeper flavor.
For a thicker gravy, stir in a cornstarch slurry (1 tablespoon cornstarch + 2 tablespoons cold water) and cook uncovered on high for 15 minutes at the end.
Store leftovers in the refrigerator for up to 4 days or in the freezer for up to 3 months.