Preheat & Prep: Preheat your oven to 300°F (150°C). Lightly grease a 9x13-inch glass casserole dish with a bit of oil or cooking spray to prevent sticking.

Add Beef: Spread the raw beef stew meat evenly in the bottom of the glass casserole dish, breaking up any pieces that are stuck together so they cook evenly.

Mix Sauce: In a medium bowl, whisk together the condensed cream of mushroom soup, dry onion soup mix, and water until mostly smooth. It doesn’t need to be perfect—just well combined so the flavors are distributed.

Pour Sauce: Pour the cream of mushroom soup mixture evenly over the raw beef stew meat in the glass casserole dish, making sure all the meat is coated. Use a spatula or spoon to gently nudge the sauce into the corners and around the edges if needed.

Cover: Cover the glass casserole dish tightly with aluminum foil to trap moisture and help the beef become tender as it bakes.

Bake: Bake in the preheated oven for 2 1/2 to 3 hours, or until the beef is very tender when pierced with a fork and the sauce has thickened into a creamy gravy. Avoid uncovering frequently, as that releases steam and can slow down the cooking.

Stir: Carefully remove the casserole dish from the oven and take off the foil, watching out for hot steam. Give the beef and sauce a gentle stir to re-coat the pieces and even out the texture of the gravy.

Rest & Serve: Taste and adjust seasoning if desired (the onion soup mix is salty, so you may not need additional salt). Let the dish rest for about 5 minutes, then serve the creamy mushroom beef over mashed potatoes, noodles, or rice.