Add the oil to a large pot over medium-high heat. Add the sausage to the pot and cook for 5 minutes to brown. Stir frequently and break it up.
Next, lower the heat to medium and add the onions, carrots, and celery to the pot. Cook with the sausage for 2-3 minutes, stir often.
Add the garlic, oregano, and black pepper. Stir to mix well, and cook for 1-2 minutes, stir often.
Add the lentils, broth and water. Raise the heat, bring to a boil. Lower the heat, cover. Simmer for 15 minutes or until they start to soften. Stir occasionally.
Add the potatoes. Bring it back to a boil, then lower the heat and cover. Cook for 15-20 minutes, or until the potatoes and the lentils are tender.
Add the spinach and stir until it wilts. Taste and add salt if needed. As a reference we added 1 teaspoon to ours.