In a large skillet over medium heat, melt 3 tablespoons of butter. Add the diced Honeycrisp apples, 3 tablespoons brown sugar, and 1.5 teaspoons cinnamon. Cook for 5 to 6 minutes, stirring occasionally, until the apples are lightly tender and glistening in aromatic syrup. Remove from heat and allow to cool slightly.

Grease a 9x13-inch ceramic baking dish with butter. Spread half of the cubed brioche bread across the bottom. Scatter the sautéed cinnamon apples and their buttery pan juices evenly over the bread, then top with the remaining cubed brioche.

In a large bowl or glass pitcher, whisk together the eggs, whole milk, heavy cream, pure maple syrup, vanilla extract, cinnamon, nutmeg, and sea salt until completely smooth. Slowly and evenly pour the custard all over the cubed bread, pressing down gently with a spatula so every cube absorbs the custard. (At this stage, you can bake immediately after a 20-minute rest, or cover and refrigerate overnight for an effortless morning bake).

Preheat oven to 350°F (177°C). In a small bowl, combine the brown sugar, flour, chopped pecans, and cinnamon. Cut in the cold butter cubes using a fork or pastry blender until pea-sized crumbles form. Scatter the streusel evenly over the casserole. Bake uncovered for 42 to 48 minutes until the top is puffed, golden brown, and crisp, and the custard center is set (registers 160°F/71°C). Dust with powdered sugar and serve warm with maple syrup.