Soak your cashews in just boiled water while you assemble your ingredients. You can do also do this 8-12 hours in advance with room temperature water.
Slice onion into half-moon slithers and chop up your leek into chunky rounds.
Drizzle olive oil into a frying pan and add your chopped onion and leeks, season with salt and sauté on a medium heat for 8-10 minutes until they’ve softened.
In a high-speed blender add all the ingredients for the creamy sauce and blend until smooth. Check for seasoning and adjust to your liking. Extra nutritional yeast will make it even cheesier.
Break your cauliflower into small bite sized florets and add them to an oven dish, along with the cauliflower leaves if using, and the leek and onion mix. Pour over your creamy sauce and mix it all together well. A little dried or fresh thyme is a lovely addition here if you have it.
Wrap your oven dish tightly in foil and add to a pre-heated oven at 200°C/400°F/gas mark 6 for about 45 minutes or until the cauliflower pieces are tender to your liking.
Top with an additional sprinkle of nutritional yeast before serving.
Alternatively grate over some vegan parmesan or breadcrumbs and pop back until the grill for 5 minutes to crisp up; if your breadcrumbs are quite dry, give the top of your gratin a drizzle with olive oil before grilling.