Brown the beef and onion for 9 minutes until the beef reaches 160°F, drain, and transfer to a 6-quart crockpot.
Brown the mushrooms in the butter in 2 batches for 6 minutes per batch and add them with the garlic.
Whisk the broth, Worcestershire, mustard, salt, pepper and flour until smooth and pour it into the crock.
Cook covered on low for 5 hours, and cook the egg noodles separately near serving time.
Switch off the crockpot, temper the sour cream with 1 cup hot gravy, stir it back in, and spoon the stroganoff over the noodles.