Preheat oven to 375 degrees and grease a 9×13 baking pan.
Add bread cubes to the baking pan – generously covering the bottom up about 1.5 inches.
In a mixing bowl, whisk eggs, milk, and 1/2 teaspoon cinnamon. Pour over the bread and push the bread down to soak up the liquid. Add or remove bread cubes, as needed, to maintain a proper proportion to the liquid.
Add the apples and 2 tablespoon butter to a large skillet over medium-low heat. Sprinkle with 1/4 teaspoon 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 tablespoon brown sugar depending on sweetness preference. Continue cooking with cover off until tender and slightly caramelized. Be careful not to cook for too long so as to get mushy. 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. If a few slices are sitting on top, that’s okay.
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 days and reheat in the microwave.