Preheat the oven to 350 degrees. In a saucepan, melt the butter. Add the leek and cook over moderately low heat, stirring, until softened. Add the celery root, broth, and water. Bring to a simmer; cook until the celery root is tender, 20 minutes. Puree the soup in a blender; return to the saucepan and stir in the cream. Season with salt and pepper.

Meanwhile, remove the peel and white pith from the clementines. Working over a skillet, cut between the membranes to release the sections. Squeeze the juice from the membranes into the skillet; add the shallot, vinegar, and mustard seeds. Cook over moderately high heat, stirring, until most of the juice has evaporated and the fruit has started to break down, 2 minutes. Let the relish cool to room temperature. Stir in the tarragon and season with salt. Spread the relish over the toasts and serve with the soup.