Grease a 9x13-inch ceramic baking dish with butter. Cut your brioche bread into 1-inch cubes. If bread is very fresh, spread cubes on a baking sheet and toast at 300°F (150°C) for 10 minutes to dry slightly.

In a medium skillet, melt 2 tablespoons butter over medium heat. Add sliced apples, 2 tablespoons maple syrup, and 1 teaspoon cinnamon. Sauté for 4 to 5 minutes until just beginning to soften. Remove from heat.

Spread half the brioche cubes evenly into the bottom of the prepared baking dish. Scatter the sautéed apples over the bread, then top with the remaining brioche cubes.

In a large mixing bowl or pitcher, vigorously whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla extract, ground cinnamon, nutmeg, and salt until smooth and fully emulsified.

Slowly pour the custard evenly over the entire dish, gently pressing down on the bread cubes with a spatula to ensure every piece absorbs the liquid. Let rest on counter for 15 minutes (or cover and refrigerate overnight!).

Preheat oven to 350°F (175°C). In a small bowl, combine flour, brown sugar, rolled oats, and cinnamon. Cut in cold cubed butter using a pastry cutter or your fingers until crumbly pea-sized clusters form. Toss in chopped pecans if using.

Scatter the streusel topping generously and evenly across the top of the soaked casserole.

Bake uncovered for 42 to 48 minutes until the top is deeply golden brown, the streusel is crisp, and the center is set (a knife inserted into center comes out clean without wet egg).

Let the casserole rest for 10 minutes before slicing to allow the custardy center to set perfectly. Dust with powdered sugar, drizzle with warm pure maple syrup, and serve with fresh whipped cream or mascarpone!