Preheat the oven to 350 degrees F.
Drain the pineapple juice into a liquid measuring cup; you should have 1 1/2 cups. Whisk in the sugar and cornstarch until the sugar dissolves. Spread the pineapple chunks in the bottom of a 13-by-9-inch baking dish and pour over the pineapple juice mixture, stirring to coat the chunks evenly.
In a large bowl, stir together the crushed crackers, cheese and butter, then sprinkle the topping evenly over the pineapple. Bake the casserole until the topping is golden brown, the cheese is melted and the pineapple is bubbling in the center of the dish, 35 to 40 minutes. Let the casserole cool for 5 minutes, then serve warm.
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