Heat the olive oil and bacon fat over medium heat in a large, heavy pot and add the carrots, celery, onion and garlic. Add a big pinch of salt. Cook until softened, about 5 minutes.
Stir in the lentils, tomatoes and 4 cups water. Add the thyme and rosemary and a couple more pinches of salt. Bring to a boil and then lower the heat to a simmer. Cook until the lentils are tender, 15 to 20 minutes, adding more water if necessary to cover the vegetables.
Meanwhile brown the sausages in about a teaspoon of olive oil in a small pan and then slice them into quarter moons.
When the lentils are just tender, taste the soup and add more salt if necessary. Add the kale and cook for about 3 minutes, until the kale is tender but still green. Stir in the sausage, make sure everything is heated through, and serve.