Add the vegan butter to a pot and let it melt. When it's melted add in the all purpose flour and stir it vigorously with the melted butter.
Then pour in the cans of coconut milk and the vegetable stock. Use a hand whisk to whisk out the lumps. Keep whisking until it starts to boil and then keep whisking for a few minutes after it has boiled until it thickens. It will continue to thicken as it cools, so it doesn't have to be all the way there when you remove it from the heat, as long as it has thickened considerably from when you started.
Remove it from the heat and add in the dijon mustard, nutritional yeast, onion powder, garlic powder, smoked paprika, salt and pepper and whisk in. Now your sauce is ready, you can prepare your macaroni.
Cook your macaroni according to the package directions and then rinse and drain.
Add the cooked macaroni into the pot of sauce and toss it up with the sauce.
Then transfer the macaroni and sauce to a 9x13 baking dish and smooth down.
Preheat the oven to 400°F (200°C).
Prepare the crumb topping by breaking up the 3 thick slices of toasted white bread and placing into the blender. Pulse blend until you have crumbs. Then transfer to a mixing bowl, and add in the melted vegan butter and toss up the crumbs with the melted butter.
Place the breadcrumbs evenly over the top of the macaroni cheese.
Bake for 20 minutes until the topping is golden brown and crispy.