To a large skillet over medium-low heat, add the 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 you used sweet or tart apples). 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.
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. (see photo)
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.
Preheat oven to 375 degrees and grease a 9x13 baking pan.