Preheat the oven to 350 degrees F
Slice the challah bread into about 1 inch cubes
Lightly grease a 9x13 inch baking dish and spread the cubed bread in the dish
In a large bowl, whisk together the eggs
Pour the canned coconut milk into a blender and blend until smooth or use a separate container and blend with an immersion blender
Add the soy milk, blended coconut milk, vanilla, cinnamon, and nutmeg to the eggs and whisk until full combined
Pour the liquid custard mixture over the bread in the baking dish, evenly distributing it over the cubes. You can press the bread slightly into the wet ingredients once it's all in the pan in order to moisten all of the bread.
Cover the dish with tin foil and refrigerate for 1 hour or up to overnight (you can skip refrigeration and bake right away if short on time)
Bake the casserole at 350 degrees F for 25 minutes covered with tin foil and then bake for an additional 20 minutes uncovered.
Slice and serve with syrup and your favorite toppings