Preheat your oven to 350°F (175°C). Grease the baking dish with a little butter or non-stick spray.
Spread the bread cubes evenly in the bottom of the prepared dish. Scatter the sliced apples over the bread, distributing them evenly.
In a mixing bowl, whisk together the eggs, milk, sugar, cinnamon, vanilla extract, and salt until well combined and slightly frothy.
Pour the custard mixture evenly over the bread and apples, pressing down gently to help the bread soak up the custard.
In a small bowl, combine the 1/4 cup brown sugar, flour, oats, and remaining butter and brown sugar. Mix until crumbly and resembles coarse crumbs to make the streusel topping.
Sprinkle the streusel evenly over the top of the casserole, covering all the surface.
Bake in the preheated oven for about 40-45 minutes, until the top is golden brown and the custard is set around the edges with a slight jiggle in the center.
Remove from the oven and let it cool slightly before serving. The edges should be crisp and the inside soft, fragrant with cinnamon and caramelized apple slices.