Crockpot Beef Stroganoff

Note: I found this recipe from Martha Stewart. I don't nomally troll Martha for recipes, but I was trying to find a crockpot version for stroganoff. This one turned out wonderful!

Crockpot Beef Stroganoff

Ingredients:
2 pounds beef chuck, trimmed of excess fat and cut into thin slices
1 large onion, chopped (about 2 cups)
1 pound white mushrooms, trimmed and halved (or quartered, if large)
Coarse salt and ground pepper
2 tablespoons cornstarch
1/2 cup reduced-fat sour cream
2 tablespoons Dijon mustard
Cooked thin egg noodles, for serving

Directions
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.