Begin by rinsing the lentils, then place them in a saucepan with 1/34 pints (1 litre) of water (don't add any salt), bring them to simmering point and cook gently for about 30 minutes, or until the lentils are tender but not disintegrating.
Then drain them and keep the cooking liquid. Meanwhile, fry the onion and garlic in the butter and 1 tablespoon of the oil in the casserole for 8-10 minutes to soften. Next, add the tinned tomatoes, sugar, thyme and bay leaf, and some salt and freshly milled black pepper, and let it all simmer gently (without a lid) until the liquid reduces and it becomes rather thick - you may need to stir it now and then and it will take about 5-7 minutes.
Now heat the remaining tablespoon of oil in a large frying pan and brown the sausages all over in it. Then, using a draining spoon, transfer them to the tomato mixture. Add the cooked lentils and 10 fl oz (275 ml) of the liquid they were cooked in. Stir well, then continue cooking over a very gentle heat for about 30 minutes - with a lid - and, if the mixture begins to look dry, add a little more liquid.
Taste and add pepper and salt, if it needs it, before serving.
You will also need a 6 pint (3.5 litre), flameproof, lidded casserole.