Heat the oil in a large, heavy-based frying pan over a medium heat. Fry the red onions with a large pinch of salt. Cook for 30-35 mins over a medium-low heat, stirring often, until caramelised and golden.

Meanwhile, make the gremolata. Mix the lemon zest, juice, parsley and garlic with a pinch of salt, then set aside. Cook the pappardelle following pack instructions, reserving 120ml pasta water.

Add the anchovy fillets to the onions when they have caramelised and let them dissolve. Stir in the tomato purée along with 15 twists of black pepper (around 1/2 tsp) and cook for 4-5 mins until the mixture has darkened.

Add the cooked pasta to the sauce along with the 120ml reserved pasta water. Stir well to combine, then transfer to a serving platter. Spoon over the gremolata to serve.