Heat the olive oil in a large Dutch oven or stockpot over medium-high heat. Add the diced onion, celery, carrot, and bay leaves; saute for 10 minutes. Add the tomato paste, salt, and minced garlic; sauté 1 minute. Add 6 cups water and lentils; bring mixture to a boil. Partially cover, reduce heat, and simmer mixture for 25 minutes. Stir in the chopped spinach, parsley, vinegar, mustard, and pepper; cook 15 minutes. Discard the bay leaves. Ladle soup into bowls; top with cheese.
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