For the Truffle Mayonnaise
Toss ¼ cup of your favorite mayonnaise in a small bowl, add a finely chopped half truffle and mix well.
For the Pickled Mustard Seeds
In a saucepan, combine 1 cup white balsamic vinegar, ½ cup sugar, ½ water, and 3 tablespoons kosher salt. Bring to a simmer, stirring to dissolve. Pour the hot pickling liquid into a glass container over ½ cup mustard seeds. Refrigerate overnight.
For the Celery Relish
Bring a pot of salted water to a boil. Prepare an ice bath. Peel and dice small ½ cup celery root and ¼ cup celery. Blanch separately until tender, and shock both in the ice bath. Drain the celeries and combine with ¼ cup half-sour pickles, diced small, 3 tablespoons drained pickled mustard seeds (above), and 1 tablespoon white balsamic vinegar in a medium bowl. Season with salt and set aside.