Cook the onions with butter in a large stockpot until golden. Add in the leeks, carrots, celery, mushrooms, potato peelings, and the 1/3 cup water. Bring to a boil, and then simmer for 5 minutes, covered.
Add in the 12 cups of water, lentils, garlic, peppercorns, thyme, bay leaf, parsley sprigs, and salt. Bring to a boil, the simmer on low, uncovered, for about 2 hours.