Thinly slice the onion, then fry it in the oil with the sausages.
When the sausages are lightly browned and the onions are soft, add the chopped garlic (and chillies). Cook for 1 minute, whilst you drain the lentils.
Add the tomatoes, lentils and water to the pot, bring to the boil and simmer for 15-20 minutes.
Chop the sage, add to the sauce for the last five minutes of the cooking.
It's ready when the sauce is thick, slighly sticky and the lentils are soft.
Season with salt to taste and pepper before serving.