Preheat oven to 375 degrees and grease a 9x13 baking pan.
Add bread cubes to the baking pan - enough to generously cover the bottom and up about 1.5 inches. You don't want to see any bare spots on the pan, but you also don't want there to be bread overflowing out of the dish.
In a mixing bowl whisk the eggs, milk, and 1/2 tsp cinnamon. Pour over the bread and use your hands or the back of a wooden spoon to push the bread down to soak up the liquid. If you end up having too much bread after pouring on the liquid, just pick a few out. Alternatively, if you have too much liquid, add more bread cubes.
To a large skillet over medium-low heat, add the sliced apples and 2 Tbsp butter. Sprinkle with 1/4 tsp cinnamon, stir and cover to steam for a bit. When they start to get tender - about 4-5 minutes - remove cover and sprinkle with 2-3 Tbsp brown sugar (this will depend on preference and whether sweet or tart apples were used). Continue cooking with cover off until tender and slightly caramelized. You do not want them to get mushy so be careful not to keep the cover on for too long. Remove from heat to cool slightly.
Check the bread to make sure all pieces are getting well soaked. If any cubes are dry on top, flip them over with your hands to ensure all pieces are wet. Sprinkle top with remaining brown sugar - about 1/4 cup.
Next spoon 3/4 of the apples over the top of the bread and push them down into the cracks with your fingers. If a few slices are sitting on top, that is all right.
Bake on a center rack for 45 minutes to 1 hour. It will be done when the bread and apples are golden brown and the egg mixture is no longer wet. Stick a fork down in the center to check - if it's still jiggly or wet, bake for another 5-10 minutes.
Remove from oven and serve immediately. Top each slice with any remaining sautéed apples and maple syrup. Store leftovers in a covered container in the fridge for up to a couple days and reheat in the microwave; though this dish is best when fresh.