Slice, then dice the apples into smaller diced pieces then add to a pan with melted butter on medium heat. Let the apples cook for a few minutes (3-4 minutes), then add in the brown sugar and cinnamon.
Toss to combine and bring the flame to low. Continue to cook for another 2-3 minutes, then set aside.
For the French Toast:
In a medium mixing bowl or in a large measuring cup, combine together the eggs, milk, vanilla, and cinnamon using a whisk, then set aside.
Dice the brioche into cubes and toss into an oven safe baking dish. Then, pour the egg mixture all over the brioche, making sure all of the bread has been soaked and covered with the egg.
Top the brioche with the sautéed apples and bake at 375º for 20 minutes.
Notes
Using fresh bread is great, but you could also use a day old brioche. The bread will absorb the egg and milk mixture regardless, but older bread will hold up better.The french toast should be eggy and wet when it bakes, so be sure to cover all of the bread to make sure that the end result yields a moist and egg texture rather than drying out.When prepping the apples, dice them finely into small cubes so that they cook a little quicker and soften easier. To reduce the cooking time of the apples, you can cover the apples in the frying pan and lower the heat so the apple cubes do not burn.