Preheat the oven to 400°. In a large saucepan, melt the butter over moderate heat. Add the celery, leeks, onion and garlic and cook, stirring, until tender, about 10 minutes. Drain the oysters and reserve 1 cup of their liquor. Add the thyme and reserved oyster liquor to the pan and cook for 10 minutes. Add the oysters, bread crumbs and scallions and cook until the oysters are just cooked through, 3 to 5 minutes. Season with salt and pepper.