Scrub the vegetables and chop them roughly into 1-inch chunks. In a large pot, add the onion, carrots, celery, garlic, parsley, and lentils and cook them over high heat for 5 to 10 minutes, stirring frequently. Add water 1 to 2 tablespoons at a time to keep the vegetables from sticking to the pot. Add 2 quarts of water and bring to a boil. Lower the heat and simmer, uncovered for 30 minutes. Strain the stock carefully and discard the solids