Generously grease a deep 9x13-inch ceramic baking dish with butter. Cut day-old brioche loaf into 1-inch cubes and spread evenly across the prepared dish.

In a large mixing bowl, vigorously whisk together eggs, whole milk, heavy cream, brown sugar, vanilla extract, cinnamon, nutmeg, and fine salt until silky smooth.

Pour the golden egg custard slowly and evenly over the brioche cubes. Press down lightly with a spatula to ensure every bread cube is submerged. Cover tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight.

Preheat oven to 350°F (175°C). Remove casserole from refrigerator and let sit at room temperature for 15 minutes while the oven heats.

In a medium bowl, combine melted butter, dark brown sugar, chopped pecans, cinnamon, and flaky sea salt. Stir until a rich, crumbly praline topping forms.

Scatter the pecan praline mixture evenly across the top of the soaked brioche bread.

Bake uncovered at 350°F for 40 to 45 minutes, until the casserole is puffed, golden brown, and the center is set (a knife inserted in center comes out clean with no raw liquid).

Let rest for 10 minutes before slicing into 8 generous portions.

Dust lightly with powdered sugar, top with fresh berries, and serve warm with generous drizzles of pure maple syrup.