In a 5- to 6-quart slow cooker, toss beef, onion, and mushrooms with 1 ½ teaspoons coarse salt and teaspoon pepper.
Cover, and cook on low until meat is tender, about 8 hours (or on high for 6 hours).
In a 2-cup glass measuring cup, whisk cornstarch with 2 tablespoons water. Ladle 1 cup cooking liquid into measuring cup; whisk to combine. Pour into a small saucepan, and bring to a boil; cook until thickened, about 1 minute.
With slow cooker turned off, stir in cornstarch mixture, then sour cream and mustard. Serve beef over noodles; sprinkle with dill, if desired.
