Tangy Dijon-mustard vinaigrette is the perfect complement to the mellow flavor of leeks.
Bring a large pot of water to a boil. With kitchen twine, tie leeks into two bundles, with green tops at the same ends. Add bundles to boiling water. Reduce heat, and simmer until leeks are tender, about 15 minutes. Remove leeks; rinse under cold water. Drain well, and gently pat dry with paper towels. Set aside.
Make vinaigrette: In a large bowl, whisk together mustard, vinegar, garlic, and basil. Slowly whisk in oil in a fine stream. Add salt and pepper.
Gently toss leeks in the bowl with vinaigrette, coating well. Serve chilled or at room temperature.