Finely dice the onion and fry in a heavy-based pan with lots of olive oil and a generous pinch of salt until sweet and tender. When ready, finely chop the garlic and add to the onion along with the chilli flakes and the anchovies. Cook gently for a few minutes, smushing the anchovies with a wooden spoon until they melt and infuse into the oil. Then add the tomato purée and cook out for a minute before pouring in the tinned tomatoes. Rinse the tins with a splash of water and add half a tin of this tomatoey water to the pan. Simmer for 20 minutes, stirring occasionally until the sauce has thickened. At this point, drain the capers and olives and rinse under a tap. Shake dry, then add to the sauce with the butter. Mix well and continue cooking for a few minutes so they become one with the sauce; taste to check your seasoning, only adding salt carefully as many of the ingredients are quite salty. Add the sugar to balance out the acidity. Drain off the sardines, then add to the pan and gently break them apart – I don’t like to smash them up too much. Turn the heat off and crack on with the pasta. Make sure to properly season your pasta water and cook the pasta until al dente. Bring the sauce back up to heat just before it’s done. Reserve a mugful of the pasta cooking water before you strain it off. Add this little by little as you whip the sauce into the pasta. Finish with the finely chopped parsley, mix again and serve with a drizzle of really good olive oil.