This recipe is part of our superfoods menu .
Meanwhile, in a large soup pot, combine 4 quarts cold water, garlic, juice of 2 lemons, thyme, and 1 1/2 tablespoons salt. Bring to a boil over high heat. Reduce heat, and simmer about 30 minutes (while leeks soak).
Remove leeks from water; add to cooking liquid. Poach until tender, 15 to 20 minutes. Drain and pat dry.
Combine juice of last lemon and mustard in a small bowl (reserving lemon slices for garnish, optional). Whisk in olive oil, then toss over leeks. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper; top with thyme and lemon slices (if using).