Peel and finely dice the onion, carrot, and celery.
Braise the vegetables in olive oil until softened.
Rinse lentils and add to the pan with water and vinegar. Simmer until tender.
Make gremolata by mixing parsley, lemon zest, garlic, salt, pepper, and olive oil.
Stir chopped parsley stems into the lentils.
Serve with gremolata and olive oil.