Preheat oven to 400°F. Place scallops in a medium sauté pan. Add the wine and a pinch of salt. Bring to a boil, reduce heat to a simmer, cover and simmer for 4 minutes. No longer. Remove the scallops to a 1.5 quart casserole baking dish. Pour off the cooking liquid to a separate container and reserve.
Heat 3 Tbsp butter in the sauté pan on medium heat. Add the onions and cook until wilted, a couple minutes.
Slowly sprinkle the flour and add the cooking liquid over the butter onion mixture, whisking vigorously while you do so. As soon as the sauce is thickened, add it to the scallops in the casserole baking dish. Stir to fully incorporate the scallops into the sauce.
Sprinkle bread crumbs over the scallops and dot with the remaining 3 Tbsp butter. Bake in the oven at 400°F until bubbly and brown, approximately 15 minutes.