Heat the olive oil in a large pot over medium heat. When the oil is hot, but not smoking, add the onions carrots and celery to the pot.
Gently cook the vegetables for 5 minutes, while stirring frequently.
Add the garlic to the pot and cook for 1 minute, stirring constantly.
Add the smoked sausage to the pot and cook them for 2-3 minutes, stirring frequently.
Add the lentils, chicken broth and the water to the pot. Raise the heat to high and bring the broth to a boil.
Lower the heat to medium when the broth is boiling vigorously. Cover the pot and cook the lentils for 25-35 minutes, stirring occasionally.
Taste the lentils to see if they have reached the desired tenderness. If they are still a bit hard for your taste, continue cooking for 10-15 more minutes and try again. Lower the heat and stir more often. As the lentils thicken, they could start sticking to the bottom of the pot.
Lower the heat to low.
Add the spinach to the lentils and sausage. Stir until the spinach wilts.
Stir in the black pepper.
Taste the lentil soup and add salt if needed.
Serve the lentil sausage soup alone, with white rice or a nice piece of crusty bread, if desired.