Start by heating your butter or olive oil in a large pot or Dutch oven over medium heat. Add the diced white onion, carrot, and celery. Cook these vegetables for about 5 minutes, stirring occasionally, until they start to soften and the onions become translucent. Then, add the minced garlic and sauté for another minute until fragrant.
Add the ground Italian sausage to the pot, breaking it up with your spoon. Cook until the sausage is browned and cooked through, about 6-8 minutes. This step builds the savory foundation of the soup, so make sure the sausage develops a nice color.
Sprinkle in the salt and Italian seasoning, stirring to combine. Pour in the low-sodium chicken broth and the can of diced tomatoes with their juices. Stir everything together, making sure to scrape up any browned bits from the bottom of the pot for added flavor.
Rinse the black beluga lentils under cold water, then add them to the pot. Bring the soup to a boil, then reduce the heat to low and cover. Let it simmer gently for about 30-40 minutes, or until the lentils are tender but not mushy. Stir occasionally to prevent sticking.
Once the lentils are cooked through, taste the soup and adjust seasoning if needed. If you want a thicker soup, let it simmer uncovered for a few more minutes. Serve hot, garnished with freshly chopped herbs or a sprinkle of Parmesan if you like.