Heat the oil in a large pan over medium heat. Add the sausages and brown them on all sides, about 5 minutes. Remove and set aside.
Add the onion to the same pan and cook for 3 minutes until soft. Add the garlic and cook for 1 minute.
Stir in the tomato puree and thyme, cook for 30 seconds.
Add the carrots, red lentils and chicken stock. Bring to the boil.
Cut the sausages into chunks and return to the pan.
Reduce the heat to low and simmer for 30 minutes until the lentils are tender and the stew has thickened.
Season with salt and pepper and serve hot.