Arrange potatoes, carrots, and onion around the edges of a 5- to 6-quart slow cooker.

Whisk celery soup and beef stew seasoning mix together in a bowl until smooth and creamy.

Heat a saucepan over medium-high heat; cook roast until browned and seared, about 3 minutes per side. Sprinkle onion soup mix over entire roast and place roast in the center of the slow cooker, surrounding it with the vegetable mixture. Spread celery soup mixture over roast.

Cook on Low, 8 to 10 hours.

Remove roast from the slow cooker and place on cutting board; let rest for 10 minutes.

Strain remaining liquid in the slow cooker into a bowl and whisk until smooth, about 1 minute.

Slice roast and serve alongside vegetables and gravy.