Melt the butter in a skillet and cook the apples with half of the cinnamon for 5 minutes. Let them cool slightly.
Grease a baking dish and add half of the bread cubes. Layer in the apples, then cover them with the remaining bread.
Whisk the eggs, milk, maple syrup, vanilla, salt, and remaining cinnamon until well combined.
Pour the custard over the bread and press gently so the upper pieces absorb some liquid. Cover and refrigerate overnight.
Sprinkle brown sugar and pecans over the casserole. Bake at 350°F (175°C) for 45 to 50 minutes, until golden and set.
Let the casserole rest for 10 minutes before serving. Cool leftover portions completely before freezing.
