Trim and discard root end from leek; split leek in half lengthwise and rinse well under running water, flipping layers to release grit. Coarsely chop leek and place in an 8- to 10-quart pan. Add onions, carrots, celery, mushrooms, tomatoes, fennel, parsley, thyme, oregano, peppercorns, and enough water to cover vegetables (about 2 qt.). Bring to a boil over high heat, then reduce heat and simmer, uncovered, for 2 hours, adding water as needed to keep vegetables barely covered.
Line a colander with a layer of cheesecloth (or use a large, fine wire strainer); set over a large bowl. Pour stock mixture into colander and drain vegetables well; discard vegetables. You should have about 8 cups stock. If you have less, thin with water; if more, boil, uncovered, until reduced to that amount.
Nutritional analysis per 1/2 cup.