- 3 1/2 quarts water (14 cups)
- 2 whole large leeks, trimmed, halved lengthwise, sliced (about 6 cups)
- 1 pound carrots, peeled, cut into 1/2-inch rounds
- 2 medium potatoes (10 to 11 ounces total), scrubbed, diced
- 2 large unpeeled onions (1 1/4 to 1 1/2 pounds total), cut into 1-inch cubes
- 3 large celery stalks, sliced
- 2 large parsnips, peeled, cut into 1/2-inch slices
- 1 cup chopped fennel stalks and fronds (reserve bulb for another use)
- 1 cup coarsely chopped fresh Italian parsley (with stems)
- 6 garlic cloves, peeled
- 1 bay leaf
- 1 teaspoon dried thyme
- 1 teaspoon fine sea salt
- 1 teaspoon whole black peppercorns
- 3 fresh mint sprigs (optional)
- Combine all ingredients in large pot. Bring to boil; reduce heat to low, cover, and simmer until vegetables are very soft and broth is pale golden, stirring occasionally and adjusting heat if necessary to maintain low simmer, about 45 minutes. Season to taste with more salt, if desired. Strain broth through colander; discard vegetables. Strain broth again through fine-mesh strainer. DO AHEAD: Can be made 3 days ahead. Cool, cover, and refrigerate.