Grease a 9x13-inch ceramic baking dish with butter. In a medium bowl, toss the sliced apples with 2 tablespoons melted butter, 2 tablespoons brown sugar, and 1 teaspoon cinnamon until glistening and evenly coated.

Scatter half of the cubed brioche bread across the bottom of the prepared dish. Layer the spiced apple slices evenly over the bread, then top with the remaining brioche cubes.

In a large mixing bowl or 4-cup measuring pitcher, vigorously whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla extract, cinnamon, nutmeg, and sea salt until completely smooth and velvety.

Pour the egg custard slowly and evenly over the layered brioche and apples, gently pressing down on the top cubes with the back of a spatula so every piece absorbs the liquid. Cover tightly with foil and refrigerate for at least 30 minutes, or overnight for up to 12 hours.

When ready to bake, preheat your oven to 350°F (175°C). In a small bowl, prepare the streusel by combining flour, brown sugar, chopped pecans, and cinnamon. Work the cold cubed butter into the mixture using your fingertips or a pastry cutter until coarse, clumpy crumbs form.

Uncover the chilled casserole, scatter the pecan streusel generously over the top, and bake uncovered at 350°F (175°C) for 42 to 48 minutes, until the custard is set in the center and the streusel topping is golden-brown and crisped.

Remove from the oven and let rest for 10 minutes to allow the custard to set cleanly for slicing. Dust with powdered sugar, top each square with a pat of fresh butter, drizzle generously with warm pure maple syrup, and serve warm!