Soak the cashews in some just boiled water to soften while you prep the potatoes.
Preheat the oven to 195˚C (180˚C fan)/365˚F (350˚F fan)/gas mark 5 (gas mark 4 fan).
Thinly slice the potatoes and butternut squash either using a sharp knife or carefully on a mandolin. The thinner you can get them, the faster the dauphinoise will cook and the meltier and more succulent the individual pieces will become – so take your time getting the slices as thin and even as possible. Cut the red onion in half and then slice each half into thin strips.
It looks striking to stack the potato and butternut squash slices vertically in the oven dish. If you want to do it this way, add all the potato pieces first and then tuck in the butternut squash pieces in between each potato slice, and finally nestle in the onion slices and the herbs. This also works as a regular dauphinoise layering-wise so, if preferred, you can layer the slices horizontally, alternating the potato and squash and red onion. Add a little salt and pepper to each horizontal layer as you go.
Once your oven dish is stacked, you can make the creamy sauce. Add the ingredients to a high-speed blender and blend until super smooth. It will be a little runny, but will thicken up in the oven once the cornflour is cooking out. Pour the creamy sauce over the sliced potatoes and squash, hopefully covering them to about ¾ of the way up.
Sprinkle over the chopped rosemary, thyme and sage, sprinkle liberally with black pepper, drizzle with olive oil and tightly wrap the dish in foil. Bake in the oven for about 45 minutes until the potatoes are cooked through, then remove the foil, and allow the top to get golden and bubbly for 10 minutes. You can also turn on the grill briefly to help the goldening, depending on your oven.
Serve this warm with your favourite Christmas vegetable sides – it’s wonderful alongside our Mushroom Log or Ultimate Nut Roast and a drizzle of hot veggie gravy!