Hasselback your potatoes. Do this by slicing them, extremely thinly, 3/4 of the way through each potato. You do not want to touch the other side of the potato - it will fall apart. Go slowly - they are basically never perfect but ain't nobody going to notice when they're covered in cheese sauce.
Add the hasselbacked potatoes to a large pot of water, and bring to the boil. Allow them to cook for around 10-15 minutes. This will depend on the size of the potato. You want them to be cooked through, but still form and holding form. While they are cooking, preheat the oven to 180 degrees Celsius.
Once the potatoes are par boiled, drain them gently, and transfer to a baking tray. Spray liberally with olive oil, and place in the oven for around an hour. You want the tops to be reasonably browned, but not so crunchy that you can't swallow them. Been there.
When the potatoes are nearly done, melt the vegan butter in a large saucepan over a medium heat. Once it has melted, add the cornflour, and whisk speedily. Turn the heat down, and follow this immediately with the milk, continuing to whisk until the lumps are gone. It really helps to have these ingredients measured out in advance, otherwise you'll get stressed and knock something ceramic onto your tiled floor. Or something.
Add the nutritional yeast and sea salt, and continue whisking until the mixture is cheeeeezy and has thickened to a roux.
Take a large baking dish, and spread the cheese sauce across the bottom. Gently press your hasselback potatoes into the sauce, give them another spray of olive oil, and return to the oven for 15-20 minutes. After this time, you can flip the oven to grill for a few minutes to brown your cheese (although be careful of your precious potatoes) and then serve. I keep this in the fridge for as long as it manages to not get eaten, which has only been 2 days, but I'm confident they'd last longer.