1. Brown the sausage in a skillet over medium heat, breaking it into crumbles as it cooks, until no longer pink and lightly browned. Drain excess fat if necessary.

2. Add olive oil to the same skillet if needed, then stir in the diced onion, carrots, and celery. Sauté for 4–5 minutes, until the vegetables begin to soften and the onion turns translucent.

3. Stir in the minced garlic and tomato paste and cook for 1 minute, just until fragrant and the tomato paste darkens slightly in color.

4. Transfer the browned sausage and sautéed vegetables to the slow cooker. Add the lentils, diced tomatoes with their juices, dried thyme, dried oregano, bay leaf, salt, and black pepper.

5. Pour in the chicken broth and stir gently to combine all the ingredients, making sure the lentils are submerged in liquid.

6. Cover and cook on LOW for 7–8 hours, or on HIGH for 3–4 hours, until the lentils are tender and the flavors have melded together.

7. About 15 minutes before serving, stir in the baby spinach (or chopped kale) and let it wilt into the hot soup. Remove and discard the bay leaf.

8. Taste and adjust seasoning with additional salt and pepper as needed. If you prefer a thicker soup, lightly mash some of the lentils against the side of the slow cooker and stir them back in.

9. Ladle the soup into bowls and top with grated Parmesan cheese and a squeeze of lemon juice, if desired. Serve hot with crusty bread or a simple salad.