If desired, season and sear the cubed beef in oil in a large skillet over high heat until browned and caramelized on all sides.
To a Crockpot, add cream of mushroom soup, beef broth, onion soup mix, and the brown gravy mix. Whisk until combined well.
Place the beef in the Crockpot and coat with the gravy. Cook on low for 7-8 hours or high for 3-4 hours, or until the beef easily shreds apart with a fork.
If desired, add chopped vegetables or frozen vegetables to the beef mixture when 1 hour of cook time remains.
Preheat the oven to 400°F. Transfer the mixture to a 13x9-inch baking dish. Using two forks, pull the cubed beef into bite-sized pieces.
In a small bowl, whisk together cornstarch and 2 tablespoons water. Stir this mixture into the baking dish.