Lightly sweat off the vegetables in a little olive oil, colour until golden brown and add the roasted shells, tomato purée, star anise, peppercorns and lemon. Combine everything together. De-glaze the pan with the white wine, add the water and bring to the boil gently. Skim off the impurities, then simmer for ½2 to 2 hours, reducing by one-third. Remove from the heat and allow to settle. Pass through a fine sieve or chinois and use as required.