This sounds very good! The recipe comes from Favorite Brand Name Recipe Cookbook.

Mustard-Onion Chuck Roast in Foil

2 Tablespoons dry mustard
1 1/2 teaspoons water
3 pound beef chuck pot roast, 2 inches thick
2 medium onions, chunked
1/2 cup soy sauce

Blend mustard with water to make a smooth paste; cover and let stand 5 minutes. Meanwhile, place large enough piece of heavy-duty foil to wrap meat completely in shallow baking pan; place meat on foil. Sprinkle onion pieces over meat. Stir 1 Tablespoon soy sauce into mustard mixture, blending until smooth; stir in remaining soy sauce. Pour mixture evenly over beef and onions. Fold foil over meat; seal securely. Roast in 325 degrees F. oven 3 hours. Remove from oven and spoon off excess fat. Cut across grain into thin slices and serve with meat juices. Makes 6 to 8 servings.