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 1/2-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.
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