Heat your Dutch oven over medium-high heat. Add the sausage and cook, breaking it up with a wooden spoon, until browned and cooked through (about 6-8 minutes). Use a slotted spoon to transfer the sausage to a plate and set aside. Leave about 1 tablespoon of drippings in the pot.

Reduce heat to medium. Add the diced onion, carrots, and celery to the pot. Sauté for 5-6 minutes until softened, stirring occasionally. Add the minced garlic and cook for 1 minute until fragrant.

Stir in the tomato paste and cook for 2 minutes, scraping the bottom of the pot. If using flour, sprinkle it over the vegetables and stir for 1 minute to coat. (This helps thicken the soup slightly.)

Pour in about 1 cup of the broth and scrape up any browned bits from the bottom of the pot (this is where all the flavor hides!). Add the remaining broth, rinsed lentils, diced tomatoes (with their juices), Worcestershire sauce, mustard powder, thyme, black pepper, and hot sauce if using. Stir everything together.

Bring the soup to a boil, then reduce heat to low. Cover and let it simmer for 25-30 minutes, until the lentils are tender but still hold their shape. Stir occasionally to prevent sticking.

Return the browned sausage to the pot. Stir in the chopped kale or spinach and cook for 5 more minutes until the greens are wilted and the sausage is heated through.

Taste the soup and adjust seasoning with salt and pepper if needed. Ladle into bowls, garnish with fresh parsley, and sprinkle with Parmesan cheese if desired. Serve hot with crusty bread!