How to Make It
In a large saucepan, melt the butter over moderate heat. Add the shallots and cook until softened, about 3 minutes. Add the sliced celery and cook, stirring occasionally, until crisp-tender and still bright green, 4 to 5 minutes. Add the water and bring to a boil. Simmer over moderate heat until tender, about 8 minutes.
Working in batches, puree the soup with the parsley. Strain the soup into a bowl, pressing on the solids. Whisk in the crème fraîche and refrigerate until chilled. Season the soup with salt and white pepper.
Just before serving, heat 1 tablespoon of the oil in a large skillet until smoking. In a bowl, season the scallops with salt and pepper and toss with the remaining 1 tablespoon of oil. Add the scallops to the pan; cook over high heat, turning, until golden and cooked through, about 3 minutes.
Ladle the soup into bowls and mound the scallops in the center. Sprinkle with the celery leaves and serve immediately.
If you can't find Nantucket bay scallops, larger scallops can be used instead; quarter them before cooking.