Scallops seared to a glistening golden brown are a good textural contrast and have a mildly briny flavor that doesn’t dominate the delicate soup.
For the Soup:
In a medium pot, melt the butter over medium-high heat and add the onions and celery seed. Saute until the onion has softened but not colored, 8 to 10 minutes and add the celery roots.
Reduce the heat to medium, add the chicken stock, and simmer until the celery roots are tender, about 30 minutes. Add the optional cream and simmer an extra 10 minutes.
Pour the soup into a blender and puree. Strain through a medium strainer and season to taste with salt and pepper. If serving immediately keep warm over very low heat. If serving later, cool to room temperature and refrigerate.
For the Seared Scallops:
In a large skillet over medium-high heat, heat the oil until very hot but not smoking. Season the scallops with sault and pepper and saute, shaking the pan frequently, until colored, about 1 minute.
Pour the soup into warm bowls. Garnish the soup with the scallops and sprinkle with the chopped parsley.