Vegan Potato Gratin with Porcini

Place the cashews in a bowl and cover with cold water. Leave to soak for a minimum of 4 hours, or overnight in the refrigerator.

Place the dried porcini in a heatproof bowl, pour over the boiling water, and leave to rehydrate for 30 minutes. Lift the porcini out with a slotted spoon, squeeze gently, and roughly chop. Reserve the soaking liquid, pouring it carefully through a fine sieve or coffee filter, leaving any grit behind.

Peel and slice the potatoes 3mm thin, using a mandoline if available. Slice the onion finely. Pick the thyme leaves. Chop the parsley.

Preheat the oven to 200°C, or 180°C for a fan oven, or gas mark 6. Slice the top off the garlic head to expose the cloves, drizzle with 1 tablespoon olive oil, wrap tightly in foil, and roast for 40 minutes until the cloves are completely soft and golden.

While the garlic roasts, warm 1 tablespoon olive oil in a wide pan over medium heat. Add the sliced onion and a pinch of salt and cook, stirring occasionally, for 12–15 minutes until deeply golden and jammy.

Add the chopped porcini to the onion pan, stir well, and cook for 3 minutes until fragrant and any residual moisture has evaporated.

Squeeze the roasted garlic cloves out of their skins into a blender. Add the drained soaked cashews, 200ml of the reserved porcini soaking liquid, the oat milk, nutritional yeast, nutmeg, 1 teaspoon salt, and the black pepper. Blend on high for 2 minutes until completely smooth and silky.

Rub a 25 × 18cm gratin dish with the remaining 1 tablespoon olive oil. Arrange one-third of the potato slices in an even, overlapping layer. Spoon over half the onion and porcini mixture and pour over one-third of the cashew cream. Repeat with a second layer of potatoes, the remaining onion and porcini mixture, and another third of the cream. Finish with the final layer of potatoes and pour the remaining cashew cream evenly over the top, pressing the potatoes gently so they are almost submerged.

Cover the dish tightly with foil and bake at 200°C for 45 minutes until the potatoes are just tender when pierced with a knife.

Remove the foil and bake uncovered for a further 20 minutes until the top is deeply golden and the cream is bubbling and reduced at the edges.

Remove from the oven and rest for 10 minutes before serving, then scatter with fresh parsley.