Heat the oil in a saucepan, add the onion and garlic and cook for a couple of minutes until softened. Stir in the tomato purée, then add the lemon juice, sugar, chilli flakes and tomatoes. Bring to the boil, then reduce the heat and simmer for 10 minutes.
Remove the pan from the heat then, using a stick blender, whizz to a smooth purée. Return to the heat, add the prawns and cook for 3–5 minutes or until they have turned pink and are just cooked through and season well.
Meanwhile, cook the pasta in a large saucepan of salted boiling water according to the pack instructions until al dente. Drain, reserving a little of the cooking water, and return to the pan. Stir through the prawn sauce, adding a little cooking water if needed. Spoon into serving bowls, then crumble over the feta and serve sprinkled with the parsley.