Preheat oven to 375 degrees F (190 C) and grease a 9x13 baking pan (adjust number or size of pan if altering batch size).
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 2/3 of the cinnamon (1/2 tsp as original recipe is written). 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. (see photo)
To a large skillet over medium-low heat, add the apples and butter. Sprinkle with remaining 1/3 of the cinnamon (1/4 tsp as original recipe is written), 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 you used sweet or tart apples // amounts as original recipe is written // adjust if altering batch size). Continue cooking with cover off until tender and slightly caramelized. You don't 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 (as original recipe is written // adjust if altering batch size).
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's OK (see photo).
Bake on a center rack for 45 minutes to 1 hour. You'll know when it's done because the bread and apples will be golden brown and the egg mixture will no longer be 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 of days and reheat in the microwave; though this dish is best when fresh.
*Try gluten-free bread to keep this recipe gluten-free, though I didn't try it myself and can't guarantee the results. *Depending on the kind of bread you use, the amount of bread will vary. The important thing is to add enough bread to generously cover the bottom of the pan, plus a little for height. For me, that was 7 or 8 cups. *I used 2 tart and 2 sweet apples for a good mix of sweet and tart. *Nutrition information is a rough estimate.