Prepare the scallops: Heat the butter and oil in a large saute pan. Season the scallops with salt and pepper and sear 1 minute per side, to brown. Do not cook completely. Set aside.
To make the duxelles: Melt the butter in a large saute pan until foaming. Add the shallot and saute until soft. Add the mushrooms, and season with salt and pepper. Cook until the mushrooms have released all their liquid and the pan is dry, about 10 minutes. Transfer the mushrooms to a bowl and stir through the parsley.
To make the sauce: Melt the butter in the saute pan until foaming. Add the shallot and cook until soft. Stir in the flour and cook, stirring, 3 minutes. Whisk in the wine and boil down by half. Add the stock and cook, stirring, until the sauce is thick enough to coat the back of a spoon. Remove from the heat and stir in the creme fraiche. Check the seasonings, adding salt, pepper, and lemon juice, if needed.
To assemble the dish: Heat the oven to broil with the oven rack at the top. Lay 6 large scallop shells or individual gratin dishes on a baking sheet. Divide the mushroom mixture among the shells. Arrange six scallops on each, then spoon over the sauce. For the topping, scatter over the parsley and breadcrumbs, drizzle over the melted butter, and broil until golden and bubbly, 2 to 3 minutes. Serve immediately.