In a large saucepan, steam cauliflower for 5 minutes before adding the broccoli and steaming for a another 3 mins. Drain well and transfer to a large ovenproof dish.
To make the sauce, melt the butter in a saucepan over medium heat. Stir in the arrowroot for approximately 30 seconds, ensuring there are no clumps. Pour in the almond milk and whisk together for 3-4 minutes until the sauce thickens. Remove the pan from heat. Whisk in the egg yolk until well combined.
Pour the sauce over the cauliflower and broccoli. Bake for 20 minutes or until the sauce is golden. Sprinkle with gluten-free breadcrumbs before serving hot.
GLUTEN FREE BREADCRUMBS - Gluten-free breadcrumbs can be expensive so I just toast a piece of gluten free bread and chop extremely finely. This works will sprinkled over the dish AFTER you have removed it from the oven.