Soak the lentils in cold water for 12 hours. Peel the onion, then take half and put it in a pan with plenty of cold water with the lentils and a bay leaf. Bring to the boil and simmer for about an hour, adding salt only when almost ready.In the meanwhile gently crush the peeled garlic, chop the parsley and blanch the tomatoes, then once deseed and peeled, remove the cooking water, then chop the coarsely. Shell and rinse the prawn tails.In a different saucepan, put the onion, the garlic and the chilli with 3 tbsp of oil and fry until soft; add the chopped tomatoes, salt and simmer for 10 minutes. Drain the lentils, toss them in the tomato sauce, add the chopped parsley, a pinch of oregano and flavour for 2 minutes.Next, heat a dash of oil in a pan and fry the prawn tails for 2-3 minutes, season them and add them to the lentil and tomato sauce. Cook the pasta in plenty of boiling water, drain it al dente with the lentil and prawn ragù. Serve immediately.