Generously butter a 9x13-inch ceramic baking dish. Arrange half of the cubed brioche bread across the bottom of the dish in an even layer.

In a medium bowl, toss the sliced apples with 2 tablespoons melted butter, 2 tablespoons brown sugar, and 1 teaspoon cinnamon until thoroughly coated. Scatter the spiced apples evenly over the bread layer, then top with the remaining brioche cubes.

In a large bowl or glass pitcher, whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla extract, cinnamon, nutmeg, and fine salt until silky smooth and completely emulsified. Slowly and evenly pour the custard over the entire casserole, gently pressing down on the bread cubes with a spatula so every piece absorbs the liquid. Let stand for 20 minutes at room temperature (or cover and chill overnight in the refrigerator for make-ahead convenience).

Preheat your oven to 350°F (175°C). In a small bowl, combine brown sugar, flour, oats, cinnamon, and chopped pecans. Cut in the cold cubed butter using a pastry cutter or your fingertips until crumbly, pea-sized clusters form. Scatter the streusel generously across the top of the soaked casserole.

Bake uncovered for 45 to 50 minutes until the top is deeply golden, crispy, and the custard is set in the center (an inserted knife should come out clean without liquid custard). Tent loosely with aluminum foil during the last 10 minutes if the streusel browns too quickly.

Let the casserole rest on a wire rack for 10 minutes to allow the custard to settle into clean sliceable portions. Dust with confectioners sugar, slice into generous squares, and serve warm with a generous drizzle of pure amber maple syrup!