Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, whisk together the eggs, milk, heavy cream, vanilla extract, cinnamon, nutmeg, brown sugar, and salt until well combined.
Layer the torn croissant pieces and sliced bananas in the prepared baking dish.
Pour the egg mixture evenly over the croissants and bananas, ensuring everything is well soaked.
Cover the dish with plastic wrap and refrigerate for at least 30 minutes, or overnight for best results.
Remove the casserole from the refrigerator and let it sit at room temperature for about 10 minutes.
Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the center is set.
Allow to cool for a few minutes before serving. Dust with powdered sugar and drizzle with maple syrup if desired.