Finely chop the walnuts or pulse in a food processor until coarse. Do the same with the mushrooms, working in batches if needed.

Heat the oven to 225°C fan. Spread the mushrooms out in a single layer across two lined baking trays. Drizzle with olive oil and season with salt and pepper. Roast for 15 mins, then scatter over the walnuts and return to the oven for 5–10 mins until everything is golden and toasty.

Peel and finely chop the onion, garlic, celery and carrot. Heat a splash of olive oil in a large pan over medium heat. Add the onion, celery and carrot, season, and cook for 10–12 mins until soft and golden. Stir in the garlic and cook for 2 more mins until fragrant.

Add the tomato purée and harissa powder to the pan. Cook for 2–3 mins to deepen the flavour. Tip in the tinned tomatoes, then fill the empty tin with water and add that too. Crumble in the stock cube and bring to a simmer.

Stir in the roasted mushrooms and walnuts, along with ¼ of the tomato tin filled with water. Simmer for 15 mins, stirring occasionally, until thick and rich. Season to taste.

Cook the pasta in salted boiling water according to the packet instructions until al dente. Reserve a mug of cooking water, then drain.

Finely grate the parmesan. Tip the drained pasta into the sauce with a splash of the cooking water and the parmesan. Take off the heat and stir well to emulsify—the sauce should cling beautifully. Add more water if needed.

Divide between bowls and top with torn basil and extra parmesan, if you like.