Italian Beef & Pasta

3 to 4 lbs. beef chuck roast
2 onions, chopped
13 1/4 oz. can sliced mushrooms
2 26 oz. marinara pasta sauce
2 T. zesty Italian salad dressing mix
16 oz. pkg. spaghetti, cooked

Combine all ingredients but spaghetti in slow cooker. Cover and cook on low for 8 hours. Slice beef and serve over cooked pasta.