Make the Apple Filling. Heat the oil in a large skillet over medium-low heat. Add the apple slices, maple syrup, and cinnamon, and mix to evenly coat. Cook for 10 minutes, until apples are soft but not falling apart. If you are using a good nonstick skillet, you can skip the oil, but be sure to stir frequently.
Lightly grease a large rectangular baking pan (about a 13×9-inch pan) with oil or nonstick cooking spray.
Make the French Toast. Arrange the slices of French bread in the greased baking dish so the tops are facing up.
In a blender, combine the oat milk, tofu, coconut sugar, cinnamon, nutmeg, ginger, salt, baking powder, and arrowroot powder (or cornstarch). Blend on high speed until the mixture is completely smooth and no lumps remain.
Pour the oat milk mixture evenly over the bread, making sure to moisten each piece completely. Pour the apple filling on top of the soaked bread and stir to combine, or carefully layer the apples between the slices of bread, as shown in the photos.
Make the crisp topping. Mix all of the ingredients together in a bowl using a spatula or your fingertips. Sprinkle the crisp topping evenly over the French Toast.
Cover the casserole with aluminum foil and refrigerate for at least 30 minutes, or overnight.
Preheat the oven to 350°F. Remove the baking dish from the refrigerator and bake, covered, for 20 minutes. Uncover and bake another 20-30 minutes, until the crisp topping is golden browned and the casserole is completely cooked through.