Grease a 9x13-inch ceramic baking dish generously with softened butter. Arrange half of the cubed brioche bread across the bottom, scatter the diced fresh apples evenly over the bread, then top with the remaining brioche cubes.

In a large mixing bowl, whisk together the eggs, fresh apple cider, heavy cream, brown sugar, vanilla extract, cinnamon, nutmeg, allspice, and salt until completely smooth and frothy.

Pour the spiced cider egg custard evenly over the brioche cubes in the baking dish. Gently press down on the bread cubes with a spatula so every piece absorbs the custard. Cover tightly with foil and refrigerate for at least 1 hour, or overnight for best make-ahead results.

Preheat the oven to 350°F (175°C). While the oven heats, prepare the streusel: In a medium bowl, whisk together flour, oats, brown sugar, cinnamon, and salt. Cut in the cold cubed butter using a pastry blender or your fingertips until crumbly, pea-sized clusters form.

Uncover the chilled french toast casserole and scatter the cinnamon oat streusel evenly over the top layer.

Bake uncovered at 350°F (175°C) for 45 to 50 minutes, until the center is set, the top is deeply golden brown and crisp, and a knife inserted in the center emerges clean and custardy.

Let the casserole rest for 10 minutes before slicing to allow the custard center to settle cleanly. Dust with powdered sugar, drizzle generously with warm pure maple syrup, and serve warm with whipped mascarpone.