Put the olive oil and butter in a large pot. Heat to melt the butter, then add the sausage links and brown evenly on all sides. Reduce the heat to keep the butter from burning. Cook the sausage for approximately five minutes or until evenly browned. If they aren’t yet cooked through, put them in an oven-proof pan in a 350°F oven till they are done.
After removing the sausage from the skillet, add the onions, carrots, celery, leeks, and seasonings to the pot. Cook over medium-heat until the vegetables are tender, about 2 minutes. Then add the lentils and chicken stock, stir briefly and bring to a boil. Stir occasionally to prevent the lentils from sticking the pan. When the soup starts to boil,
reduce the heat to low, and simmer until the lentils are tender, approximately 40-50 minutes. While the soup is simmering, slice the sausages into half-inch slices.
When the lentils are tender, add the sausage, cream, mustard, vinegar, and ½ cup of the spinach. Stir to mix all ingredients. Return the soup to a boil, then reduce the heat and simmer for an additional five minutes.
Serve in warm bowls, garnished with the remaining spinach and Parmesan cheese. Enjoy
with rustic bread.