Prep & Butter the Dish: Grease a 9x13-inch ceramic baking dish generously with 1 tablespoon softened butter. Spread half of the cubed brioche bread across the bottom of the dish.
Sauté the Spiced Apples: Melt 2 tablespoons butter in a skillet over medium heat. Add sliced apples, 2 tablespoons brown sugar, and 1 teaspoon cinnamon. Sauté for 5 to 6 minutes until tender and caramelized. Scatter the warm spiced apples evenly over the bottom layer of brioche, then cover with the remaining brioche cubes.
Whisk the Custard: In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla extract, cinnamon, nutmeg, and sea salt until completely smooth and emulsified.
Soak the Bread: Pour the custard slowly and evenly over the bread cubes, pressing gently with a spatula so every piece absorbs the liquid. Cover with foil and refrigerate for at least 30 minutes (or overnight for effortless morning baking).
Make Pecan Streusel: In a medium bowl, combine flour, brown sugar, cinnamon, and chopped pecans. Cut in cold cubed butter with a fork or pastry cutter until crumbly with pea-sized clumps. Sprinkle streusel evenly over the top of the soaked casserole.
Bake & Serve: Preheat oven to 350°F (175°C). Bake covered with foil for 25 minutes. Remove foil and bake uncovered for an additional 20 to 22 minutes until the top is golden-brown, puffed, and a knife inserted into the center comes out clean. Rest 10 minutes, dust with powdered sugar, drizzle with warm maple syrup, and serve warm.