Grease a 9x13 baking dish with cooking spray.
Cube the bread into small 1-inch cubes and place in the prepared baking dish (make sure to butter the dish).
1 large loaf bread
In a large mixing bowl, mix together the diced apples, melted butter, brown sugar, corn starch, cinnamon, nutmeg, and lemon juice.
3 cups apples, 1 Tablespoon salted butter, 1/4 cup light brown sugar, 1/2 Tablespoon corn starch, 1 1/2 teaspoons ground cinnamon, 1/2 teaspoon ground nutmeg, 1 1/2 teaspoons lemon juice
Toss the apple mixture with the bread in the baking dish.
In a large mixing bowl, whisk together the eggs, milk, vanilla extract, and salt.
6 large eggs, 2 cups whole milk, 1 Tablespoon vanilla extract, 1 teaspoon salt
Pour the egg mixture over the bread cubes in the baking dish, ensuring all of the bread is soaked with the egg mixture.
Cover the dish and chill for 4 to 6 hours, or overnight.
When ready to bake, remove the casserole from the fridge and preheat the oven to 350°F.
In a small mixing bowl, combine the sliced butter, brown sugar, and cinnamon for the topping.
4 Tablespoons salted butter, 1/2 cup light brown sugar, 1 teaspoon ground cinnamon
Crumble or clump the topping evenly over the breakfast casserole.
Cover the baking dish with aluminum foil, and bake for about 40 minutes.
Uncover and continue baking for an additional 15-20 minutes, until the casserole is golden and cooked through.