Heat the olive oil in a large pan and cook the onion and garlic over a medium heat for 6–7 minutes, until softened. Add the tomatoes, chilli flakes, sugar, lemon rind and seasoning and bring to the boil. Cover and simmer for about 10 minutes, until thickened.
Meanwhile, cook the linguine in a large pan of boiling salted water for 11 minutes, or according to the packet instructions, until ‘al dente’. Drain, reserving 2 tablespoons of the cooking water and return to the pan.
Stir the sardines and capers into the tomato sauce for the final 1–2 minutes. When hot, add to the drained pasta along with the reserved cooking water and toss gently. Serve heaped into bowls and garnished with extra black pepper, and with basil leaves, if desired.