Make the French toast: In a medium bowl, beat the eggs. Add the milk, cream, brown sugar, vanilla, cinnamon, nutmeg, and salt and whisk to combine.
Grease a 9 x 13-inch (22 x 33 cm) ceramic or glass baking dish with butter.
Working in batches, dip the brioche into the custard to coat both sides. Arrange the brioche in the prepared baking dish, overlapping slightly. Pour the remaining custard over the top. Cover with plastic wrap and refrigerate for at least 2 hours, up to overnight.
Meanwhile, make the walnut streusel: Preheat the oven to 375°F (190°C). Line a baking sheet with a nonstick mat.
In a medium bowl, combine the flour, granulated and brown sugars, salt, and cinnamon. Work the butter into the dry ingredients with a fork until crumbly. Add the chopped walnuts and mix well.
Spread the streusel on the prepared baking sheet and bake for 20 minutes, stirring halfway, until golden brown. Remove from the oven and let cool slightly, then crumble and let cool completely before using. The streusel will keep in an airtight container for 1 week.
Unwrap the French toast. Cover with foil and bake for 25 minutes, then uncover and bake until the top is golden brown and crunchy, 20-25 minutes more. Remove from the oven and let sit for 10 minutes. Top with the walnut streusel and serve with maple syrup.