Heat vegetable oil in a Dutch oven or heavy pot over medium-high heat; fry roast in hot oil until browned on both sides, about 10 minutes.
Pour mushrooms and 1 cup beef broth over roast.
Sprinkle with onion soup mix.
Bake in preheated oven to your desired degree of doneness, or an internal temperature of 145 degrees F (65 degrees C) for medium, about 90 minutes.
Remove roast from Dutch oven to a platter, cover with a doubled sheet of aluminum foil, and allow to rest in a warm area 10 to 15 minutes before slicing.
Whisk cornstarch and remaining 1/4 cup beef broth in a bowl and stir into Dutch oven with mushrooms and pan drippings. Bring to a boil over medium-high heat; cook and stir until thickened, about 2 minutes.