Heat oven 400 degrees. Spray lg cookie sheet with non-stick cooking spray. Unroll dough onto cookie sheet, press dough into 17x12 in. rectangle. Bake 7-9 minutes.
Meanwhile, in large bowl, beat cream cheese, mayonnaise, mustard and garlic salt until smooth.Stir in waterchestnuts and crabmeat.
Spread crabmeat mixture over partially baked crust withn 1 inch of edge. Spoon small spoonfuls of pineapple topping evenly over crabmeat mixture; spread evenly. Top with cheese and sprinkle with onions. Bake 12-15 minutes until cheese is melted and golden brown