In a large saucepan, cook the shallots in ¼ cup (60 ml) of butter for about 2 min. over medium heat until translucent. Add the celery, salt and pepper, and cook for 2 to 3 min. more.
Add the wine and bring to a boil. Add the mussels and clams and cover tightly. Cook over high heat, stirring once or twice, until shells have opened, about 5 to 7 min. Discard any mussels or clams that remain closed or are cracked.
Meanwhile, in small bowl, mix remaining butter and flour until the mixture is smooth.
Remove the mussels and clams and place in a warmed serving dish, leaving the broth in the pan. Bring broth to a boil and gradually whisk flour-butter mixture into the broth, and simmer until thickened, 1 to 2 min.
Pour the sauce over the mussels and clams, and serve with bread on the side.
This dish serves 4 as hearty main course, or up to 8 as an appetizer.