Grease a 9x13-inch ceramic or glass baking dish generously with softened butter.

Cut the brioche loaf into 1-inch cubes and arrange them evenly in the prepared baking dish in a snug, uniform layer.

In a large mixing bowl or 4-cup glass pitcher, whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla extract, ground cinnamon, grated nutmeg, and fine sea salt until completely smooth and emulsified.

Slowly pour the spiced custard evenly over all the brioche cubes in the dish. Gently press down on the cubes with your hands or a spatula to ensure every piece absorbs the custard. (At this stage, you can cover and refrigerate overnight, or let sit for 20 minutes before baking).

In a medium bowl, combine the flour, packed dark brown sugar, cinnamon, chopped pecans, and pinch of salt. Add the cold cubed butter and cut in with a pastry cutter or clean fingertips until crumbly, pea-sized streusel clumps form.

Scatter the cinnamon pecan streusel evenly across the top of the soaked brioche.

Preheat oven to 350°F (175°C).

Bake uncovered for 42 to 46 minutes, until the casserole is puffed tall, the custard is fully set in the center (no jiggle), and the pecan streusel crust is deeply golden-brown and crisp.

Remove from oven and let rest on a wire rack for 8 to 10 minutes so the custard firms up slightly for clean slicing.

Dust with powdered sugar, drizzle generously with warm pure maple syrup, and serve warm with fresh berries.