Grease a 9x13-inch ceramic baking dish generously with softened butter. Slice the brioche loaf into 1-inch cubes. If the bread is very fresh, spread the cubes across a large baking sheet and let them sit at room temperature for 2 to 3 hours, or bake at 300°F (150°C) for 10 minutes until lightly dry to the touch but not browned. Transfer the bread cubes into the prepared baking dish in an even, airy layer.

In a large mixing bowl, whisk together the 8 large eggs, whole milk, heavy cream, brown sugar, pure vanilla extract, ground cinnamon, ground nutmeg, and kosher salt until completely smooth and emulsified.

Pour the spiced custard slowly and evenly all over the brioche cubes in the baking dish. Gently press down on the bread cubes with a silicone spatula so every piece absorbs the liquid. Cover the dish tightly with foil and refrigerate overnight (at least 6 hours, up to 18 hours).

The next morning, melt 3 tablespoons of unsalted butter in a large nonstick skillet over medium heat. Add the diced apples, brown sugar, pure maple syrup, ground cinnamon, and lemon juice. Cook, stirring occasionally, for 6 to 8 minutes until the apples are tender-crisp and coated in a glossy, bubbly cinnamon-caramel sauce. Remove from heat.

Preheat your oven to 350°F (175°C). Remove the chilled French toast bake from the refrigerator 20 minutes prior to baking to take off the chill. Uncover and spoon the warm caramelized apples and all their buttery pan juices evenly across the top of the soaked brioche.

Cover loosely with foil and bake for 30 minutes. Remove the foil and continue baking uncovered for an additional 20 to 25 minutes, until the top is puffed, golden brown, caramelized on the edges, and the custard center is softly set (reaching an internal temp of 165°F/74°C).

Let the French toast bake rest for 10 minutes to allow the custard to settle. Dust generously with confectioners powdered sugar, scatter toasted pecans on top if desired, and serve warm with plenty of pure maple syrup.