Put the lentils in a soup pot, cover with 6 cups cold water, and bring to a boil. Skim off any foam that rises, then add the onion, garlic, bay leaves, carrot, celery, and potato, and 1 1/2 teaspoons salt. Lower the heat and simmer until tender but still a little firm -- they shouldn't be mushy -- about 25 minutes. Remove the bay leaves, taste for salt, and season with pepper. Stir in the oil and add a few drops vinegar to bring up the flavor. Garnish with parsley and serve.
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