Grease a 9x13-inch baking dish generously with butter. Spread half of the cubed brioche bread evenly across the bottom of the dish.

In a large skillet over medium heat, melt 3 tablespoons of butter. Add sliced apples, brown sugar, cinnamon, and nutmeg. Sauté for 6 to 8 minutes until apples are slightly tender and coated in a glossy caramel sauce. Spoon the caramelized apples over the bottom layer of bread, then top with the remaining brioche cubes.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla extract, and salt until completely smooth and frothy.

Pour the custard mixture evenly over the brioche and apples, gently pressing down on the top bread cubes with a spatula so every piece absorbs the liquid. Let sit for 20 minutes (or cover and refrigerate overnight).

Prepare the streusel: In a medium bowl, combine flour, brown sugar, chopped pecans, and cinnamon. Work the cold butter cubes into the mixture with your fingertips or a pastry cutter until crumbly and pea-sized. Scatter evenly over the casserole.

Preheat oven to 350°F (175°C). Bake the casserole uncovered for 38 to 42 minutes, until golden brown, puffed in the center, and a knife inserted comes out clean.

Allow to cool for 5 minutes before slicing. Dust with powdered sugar, drizzle with warm pure maple syrup, and serve warm.