Preheat the oven to 350°F and lightly spray a 9×13-inch baking dish with cooking spray.
Melt the light butter in a skillet over medium heat. Add the diced apples, brown sugar, and 1/2 teaspoon of cinnamon. Cook for 4–5 minutes, stirring occasionally, until the apples begin to soften.
Spread the cubed bread evenly in the prepared baking dish. Spoon the cinnamon apples over the bread.
In a medium bowl, whisk together the eggs, skim milk, vanilla extract, remaining cinnamon, nutmeg if using, and sugar-free maple syrup.
Pour the egg mixture evenly over the bread and apples. Gently press down on the bread to help it absorb the custard.
Let the casserole rest for 10 minutes. Bake uncovered for 40–45 minutes, or until the center is set and the top is lightly golden.
Let cool for 5–10 minutes before slicing. Serve warm with additional sugar-free maple syrup if desired.