Cook sausages first. Remove from saucepan. Fry off onions, carrots and any other veg you have and may like to add (courgettes, red peppers, leeks, cauliflower, broccoli are all good). Add tin of chopped tomatoes, some herbs (herbes de provence are good, otherwise parsley and oregano are my faves) and a couple of handfuls of red split lentils. Add some Worcestershire sauce. Add the sausages back in.
Add enough water or vegetable stock to cover everything. Bring to the boil, then lower heat, cover and simmer until the lentils and veg are cooked. Stir occasionally.
It's also nice if you add some chard or spinach near the end of cooking.