Preheat oven to 350°F.

Combine salt, pepper and cornstarch. On a large plate, season bottom round roast with cornstarch mixture, reserving excess that doesn't stick to roast.

Heat oil in Dutch oven on stovetop over medium-high heat.

When oil begins to smoke, sear roast on all sides until brown. Remove roast from pan and set aside on a clean plate.

Add mushrooms and sear over medium heat until browned.

Stir in onions to soften, 2-3 minutes.

Whisk flour and stock together in a measuring pitcher, creating a slurry.

Add slurry and any reserved cornstarch mix to mushroom onion mixture. Bring back to a boil then turn off stovetop heat.

Add roast to center of Dutch oven with mushrooms, cover and place in preheated oven; braise 2 1/2-3 hours until tender enough to separate easily with the side of a fork.

Cook noodles according to package directions.

Remove roast from oven to a clean cutting board; tent with foil.

Stir sour cream into braised mushroom mixture.

Pull pot roast apart with fork and serve over noodles with mushroom onion sauce.