Heat the oil in a frying pan and fry the sausages quickly until browned on all sides. Transfer to a flameproof casserole or large saucepan.
Reduce the heat and add the onions to the pan. Fry until softened and beginning to turn golden. Stir in the garlic, then add to the sausages.
Add the lentils, stock, tomatoes, tomato purée and herbs, if using to the casserole. Bring to the boil. Season with salt and pepper.
Reduce the heat, cover and simmer for 40 minutes.
Freezes well for up to 3 months. Defrost fully before reheating until piping hot.Nutrition information is approximate and is meant as a guideline only. (It does not include seasoning with salt and pepper.)