1. Heat the oil in a shallow flameproof casserole dish, add the onion and cook over a medium heat for 10 minutes or until very tender and golden. Put the lentils into a sieve, rinse under cold water then drain.

2. Stir the garlic and lentils into the pan and stir over the heat for 2 minutes. Pour in the stock, add the thyme and bay leaf and the chilli flakes if using. Cover and simmer for 25 minutes.

3. Meanwhile, trim and slice the leeks, wash carefully to remove all the grit.

4. Stir the chopped tomatoes and leeks into the lentils, cover and cook for a further 10 minutes by which time the lentils and leeks should be tender. Remove the bay leaf and season if needed.

5. Cut each pancetta slice into half, place under a hot grill and cook until crisp. Arrange three pieces on top of each portion of the braised lentils with leek.