Add the beef to the oil and season with the sage; cook the beef until browned on all sides; drain.
Place beef, potatoes, carrots, mushrooms, onion, celery seed, Worcestershire sauce, pepper, and beef bouillon in a slow cooker; season with salt.
Pour enough water over the mixture to cover. Set slow cooker to HIGH and cover. Cook for 4 to 5 hours, stirring occasionally.
Whisk together the cornstarch and warm water in a small bowl until smooth; stir through the stew. Allow the stew to cook until thickened, 15 to 20 minutes.