Prepare the Oven and Dish: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish to ensure nothing sticks.

Combine the Ingredients: In a large bowl, mix together the drained pineapple chunks and crushed pineapple with sugar, flour, and shredded Cheddar cheese. This creates a rich and cohesive base for your casserole.

Assemble the Casserole: Transfer the pineapple and cheese mixture into the greased baking dish, spreading it evenly.

Prepare the Topping: In a small bowl, mix the melted butter with the crushed Ritz crackers. This mixture will form a crispy, flavorful topping for the casserole.

Bake to Perfection: Sprinkle the cracker mixture evenly over the pineapple base. Bake in the preheated oven for 25-30 minutes, or until the topping is golden brown and the casserole is bubbly.