Grease a 9x13-inch ceramic or glass baking dish generously with 1 tablespoon of softened butter. Cut the brioche loaf into 1-inch cubes and arrange them evenly in the prepared dish, creating an even, fluffy layer.

In a large mixing bowl, vigorously whisk together the eggs, whole milk, heavy cream, brown sugar, vanilla extract, ground cinnamon, nutmeg, and salt until completely smooth, aerated, and fragrant.

Slowly pour the custard mixture evenly over the brioche cubes in the baking dish. Gently press down on the bread cubes with a silicone spatula so every piece absorbs the rich custard. Let rest at room temperature for 15 minutes (or cover tightly and refrigerate overnight for make-ahead convenience).

In a medium bowl, combine brown sugar, chopped pecans, flour, and cinnamon. Add the cold cubed butter and cut in using a pastry blender or your fingertips until crumbly, pea-sized streusel clusters form. Scatter the streusel evenly across the top of the soaked bread.

Preheat your oven to 350°F (175°C). Bake the casserole uncovered for 35 to 40 minutes until puffed, deeply golden-brown on top, and a knife inserted into the center emerges clean without raw liquid custard. Let rest for 10 minutes before serving.

Dust the top lightly with powdered sugar using a fine mesh sieve. Scatter fresh raspberries and blueberries over the casserole, and serve warm with generous drizzles of heated pure maple syrup.