Preheat oven to 325 degrees. In a large bowl, combine mayonnaise, mustard, cream cheese, sour cream, lemon juice, garlic salt, Old Bay, and half of the shredded cheese until thoroughly blended. Gently fold in the crab meat. Mix in splashes of milk until it reaches a creamy consistency.
Place in a well-greased baking dish and top with the remaining shredded cheese. Bake at 325 degrees for 30 minutes.