The night before, soak the cashew nuts in water and leave overnight.
To make the crumble, tip the oats and flour into a bowl, make a well in the centre and pour in the coconut oil. Draw the dry ingredients in until everything is well coated and resembles chunky breadcrumbs. Tip the mixture onto a baking sheet and put in the fridge to firm up – this will make your topping extra crunchy.
Heat oven to 200C/180C fan/gas 6. Steam the carrots and potatoes together for 5 mins until softened, then transfer to a food processor. Drain the cashews and add them too, along with 2 tbsp oil and the rest of the sauce ingredients except for the leeks. Blitz the mixture, season and set aside.
Heat the remaining oil in a large frying pan over a medium heat. Add the leeks and a large pinch of salt, then fry for 15 mins until softened. Spoon in the cheese sauce – if it is very thick, add a splash of water to thin it. Transfer the mix to an ovenproof dish, top with the crumble and bake for 30 mins until piping hot. For a crispy topping, put under the grill to finish. Can be assembled in advance and kept in the fridge to cook when needed.