In a small saucepan cook the butter, brown sugar and corn syrup over medium heat, stirring frequently, until smooth and tawny. Pour the mixture in an even layer into a 12"x 8" baking dish. (If you use a different size casserole pan make sure to adjust the cooking time.)
Tear or cut the bread into cubes about 1" in size . Put the bread into your casserole pan in an even layer.
In a medium-large measuring cup with a spout whisk together the eggs, milk, cream, vanilla, and salt until combined well. Pour evenly over the bread.
Preheat the oven to 350 F.
Peel the apples, quarter them and remove the cores. Cut each quarter into five slices. Put apples in a large nonstick skillet on medium-heat with the 3 Tbsp. butter and the 1/2 tsp. cinnamon. When the apples start to sizzle stir and then cover with a lid. Cook for three minutes stirring once. Remove the lid and add the 3 Tbsp. brown sugar. Stir to coat, then spoon the apples /sauce over the casserole. (You can start making the topping while this cooks.)
In a medium bowl, combine the ingredients for the crisp topping. Use your hands to scatter the topping over the casserole.
Bake casserole, uncovered, in middle of oven until puffed and the top has browned 40 - 50 minutes. At 40 minutes the casserole will be 160F when checked with an internal thermometer and the French toast will have a softer texture. At 50 minutes the casserole is a bit firmer. Doneness is personal preference.
Serve hot casserole with, butter, favorite syrup or whipped cream as desired.
Notes
Overnight French Toast: You can assemble everything the night before if you like and refrigerate the casserole. After spooning on the apples cover the casserole with plastic wrap and refrigerate it until the next morning. In the morning add the topping and bake as directed. Remember it will take longer to bake it cold. One option is to allow it to come up closer to room temp on the counter but either way plan on more baking time to reach the desired level of doneness.