8 baby leeks, white and light green parts only, trimmed and halved lengthwise
2 teaspoons yellow mustard seeds
2 tablespoons extra-virgin olive oil
1/4 cup small sage leaves
1 tablespoon white wine vinegar
1 teaspoon Dijon mustard
Salt and freshly ground black pepper
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.
The cooked leeks can be refrigerated overnight. Serve lightly chilled or at room temperature and dress just before serving.