Spread lentils evenly on a cookie sheet.
Pick out any small stones or foreign objects.
Rinse thoroughly in a colander with cold water.
In a large kettle, heat olive oil over medium-high heat.
Add onions, celery, and carrots, sautéing for 5 minutes.
Add lentils, garlic, bay leaf, pepper, oregano, tomato paste, and 8 cups cold water (or enough to cover).
Bring to a boil.
Cover the kettle and simmer over low heat for 1 hour.
Add more water if the soup becomes too thick.
Add salt and vinegar to the lentil mixture, stir well, and cover.
Continue simmering, stirring occasionally, for an additional 30 minutes or until lentils are tender.
Stir in chopped parsley and serve the soup in individual bowls.