Coat the baking dish with some vegan butter or oil spray. Arrange the slices of bread in the casserole, overlapping them like shingles. Fill in any empty spaces with the smaller end slices.
In a blender, place the banana, cashews, nondairy milk, cinnamon, nutmeg, sugar, flax meal, aquafaba and vanilla. Blend until smooth and creamy.
Pour the custard over the bread evenly and set aside for 15 minutes, while you make the walnut topping.
Set the oven to preheat to 350 degrees.
To make the topping, place the walnuts in a bowl with the sugar, butter, cinnamon and nutmeg. Mix with your hands until the butter is dispersed through the mixture. Sprinkle it evenly on top of the casserole.
Bake the casserole for 40 minutes. Serve the French toast with maple syrup, if desired.