Heat the oil in a shallow pan and lightly brown the sausages all over. When they are almost cooked and nicely coloured lift out and set aside, leaving the fat in the pan.
Stir the onions into the fat and leave to cook over a low heat for about 25 minutes or until the onions are soft and a deep amber in colour. Add the garlic after 20 minutes.
Meanwhile cook the lentils in deep boiling water for about 20 minutes until tender.
When the onions are ready stir in the onions and cook for 7-10 minutes until the tomatoes have collapsed. Stir in the harissa paste.
Slice the sausages into thick pieces and add to the onions. Drain the lentils and stir in, then check seasoning and serve.