Grease a 9x13-inch baking dish with butter. Set aside.
Melt butter in a large saucepot over medium heat. Add sliced apples, brown sugar, maple syrup, and cinnamon, stirring to combine. Bring a to simmer and continue to cook until apples are softened and syrup is slightly thickened, about 12-15 minutes. Remove from heat and let cool slightly.
Pour half of the apples with the liquid in the bottom of the prepared baking pan.
Slice challah into 1-inch thick slices. In a large bowl, whisk together eggnog, eggs, vanilla, and nutmeg until well combined. Dip challah slices in batter, letting each slice sit in the batter for 10 seconds. Arrange the bread slices on top of the apples in the baking dish. Pour remaining eggnog batter evenly on top of the bread.
Spoon remaining apples on top of the bread. Cover and let rest in the fridge overnight, about 8-12 hours.
Preheat oven to 325 degrees F.
Cover the french toast with aluminum foil and bake for 35 minutes.
Meanwhile, make pecan streusel. Mix all-purpose flour with brown sugar, chopped pecans, and cinnamon in a medium bowl. Use a pastry cutter or two forks to cut in the butter until well combined. The resemble should resemble coarse sand.
Remove french toast from the oven and remove the cover. Increase oven to 375 degrees F.
Sprinkle streusel on top and bake for another 5-7 minutes or until golden brown. Remove from the oven and serve warm.