Preheat the oven to 350°. Spread the walnut pieces on a baking sheet and toast in the oven for about 7 minutes, until lightly browned. Lower the oven temperature to 300°.
Remove the Camembert from the box and unwrap it. Put the cheese back in the bottom half of the box and set it on a baking sheet. Bake for about 10 minutes, until soft.
Meanwhile, in a large skillet, heat the walnut oil. Add the mushrooms and season with salt and pepper. Cover and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Uncover and cook, stirring, until lightly browned, 3 minutes longer. Add the shallot and cook until softened, 2 minutes. Stir in the parsley and sage; season with salt and pepper.
Invert the Camembert onto a platter. Stir the walnuts into the mushrooms and spoon over the cheese. Serve with the toasts.