Preheat oven to 375F.
Arrange bread in a 9x13 oven-proof casserole dish.
In a large bowl whisk together eggs, milk, vanilla and 1 tsp of cinnamon and pour over bread, pressing bread down to soak up all the liquid.
Meanwhile, melt butter in a non-stick medium frying pan over medium-high heat. Add apples, and cook for 4 minutes, stirring occasionally. Add in walnuts (if using), brown sugar and remaining 1/2 tsp of cinnamon, cook for another 2 minutes.
Pour apples on top of bread, pressing down with a spoon to arrange evenly and press apple slices into the cracks.
Bake in the centre of the oven for 20-25 minutes.