Poach leeks Bring 5 quarts water to a boil in a large pot with the thyme, lemon juice, and 2 teaspoons salt. Add leeks and reduce to a simmer, then cook until tender, 6 to 10 minutes (or 12 to 15 minutes if medium-size). Remove carefully with tongs and drain on paper towels.
Marinate Put leeks in a shallow dish and pour vinaigrette over. Let stand at room temperature for 30 minutes or refrigerate, covered, up to 1 week.
