Begin by setting your oven to preheat at 350°F (175°C). Lightly coat a medium-sized baking dish with butter or non-stick spray.
In a sizable mixing bowl, thoroughly whisk together the crushed pineapple with its liquid, eggs, granulated sugar, and flour until a smooth, cohesive mixture forms.
Pour in the melted butter and stir to incorporate. Then, carefully fold in the cubed bread, ensuring the pieces are well-covered and begin to absorb the liquid.
Transfer the combined mixture into the greased baking dish, spreading it out evenly. If using, sprinkle ground cinnamon over the top for a hint of warmth and spice.
Bake in the center of the preheated oven for 45 to 50 minutes, or until the top develops a golden crust and the center is firm to the touch.
Allow the casserole to rest for several minutes before serving. Best enjoyed warm.