Preheat oven to 350°F. Mix cream cheese, mayonnaise, Old Bay Seasoning and ground mustard in medium bowl until well blended. Add crabmeat; toss gently.
Spread in shallow 1 ½-quart baking dish. Sprinkle with Cheddar cheese and additional Old Bay Seasoning, if desired.
Bake 30 minutes or until hot and bubbly. Serve with assorted crackers or sliced French bread.
