Beef marrow bones are filled with homemade croutons and a trio of sauteed mushrooms -- chanterelle, hen of the woods, and porcini -- for the bottom layer of our Cote de Boeuf Rossini.
Bring a medium pot of water to a boil. Add marrow bones and boil 10 minutes. Remove from pot and set aside to cool. Remove and discard any excess meat and fibers. Let stand until completely dry.
In a medium skillet, heat 2 tablespoons clarified butter over medium. Add mushrooms and season with salt and pepper. Cook, stirring occasionally, until tender.
In another medium skillet, heat remaining 2 tablespoons clarified butter over medium. Add bread and cook, stirring frequently, until brown on all sides, about 5 minutes. Transfer to a paper towel-lined plate to drain.
In a medium skillet, combine marrow, sauteed mushrooms, and croutons over medium heat. Toss to combine; cook 2 minutes. Stir in fines herbes. Mound mixture into each marrow bone.