In a 5- to 6-quart slow cooker, toss beef, onion, and mushrooms with 1 1/2 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.
If you decide to use an oven instead of the slow cooker, preheat the oven to 350 degrees. Follow step 1, using a 5-quart Dutch oven or heavy pot with a tight-fitting lid instead of the slow cooker. Add 2 cups water, and cover. Transfer to oven; cook until beef is tender, about 2 hours. Proceed to step 2.