For roasted vegetable broth: Place a rack in the middle of the oven and preheat the oven to 500°F.

Place the garlic, onions, carrots, tomatoes, and leeks in a large roasting pan. Add the olive oil and toss to coat. Roast for 15 minutes. Turn the vegetables and roast for 15 more minutes.

Move the vegetables around in the pan and roast them for 10 more minutes, or until the vegetables are all nicely browned and the tomatoes are collapsing.

Transfer the roasted vegetables to a tall narrow stockpot. Add the spinach, celery leaves, parsley stems and bay leaves to the pan. Place the pan on the stovetop and add 7 cups water. Bring to a boil, scraping up any browned bits from the sides and bottom of the pan with a wooden spoon. Pour this liquid over the vegetables in the stockpot.

For plain vegetable broth: Place all the ingredients plus 8 cups water in a tall narrow stockpot. Bring to a boil. Lower the heat and simmer, partially covered, for 45 minutes.

For either method: Strain the broth through a damp-cloth-lined sieve. Use immediately or refrigerate for up to 3 days or freeze.