Heat half the oil in a saucepan, add the chopped garlic, onion and chilli, and cook for 1–2 minutes, until just softened. Add the tomatoes, measured water, lemon rind and sugar, then season to taste and simmer gently for 15–18 minutes.
Cook the pasta shells in a large saucepan of lightly salted boiling water for 11 minutes, or according to packet instructions, until al dente.
Meanwhile, heat the remaining oil in a large frying pan and cook the aubergine for about 8 minutes, turning occasionally, until golden. Transfer to the simmering pan of tomato sauce for the remaining cooking time.
Stir the mussels into the tomato sauce for the final minute, cook until thoroughly heated through, then spoon over the drained pasta to serve.