How to Make It
Using kitchen twine, tie the halved leeks back together. In a large skillet of boiling salted water, boil the leek until just tender, about 4 minutes. Drain the leeks and discard the twine. Pat the leeks dry on paper towels.
Wipe the skillet dry. Add the mustard seeds and cook over moderately high heat until they darken slightly, about 1 minute. Transfer the mustard seeds to a plate. Heat 1 tablespoon of the olive oil in the skillet. Add the sage leaves and cook over moderately high heat until crisp, about 1 1/2 minutes. With a slotted spoon, transfer the sage leaves to the plate.
In a small bowl, combine the remaining 1 tablespoon of olive oil with the vinegar and Dijon mustard and season with salt and pepper. Arrange 4 leek halves on each plate, cut sides up, and spoon the vinaigrette on top. Sprinkle the mustard seeds and sage leaves over the leeks and serve.