How to Make It
Preheat the oven to 325°. In a large Dutch oven, heat the olive oil until shimmering. Season the beef shanks with salt and pepper and brown them in the oil over moderately high heat, about 2 1/2 minutes per side. Transfer the shanks to a plate.
Add the herb bundle to the pot along with the mushrooms, onion, carrot and celery. Cook, stirring, until the vegetables begin to soften, about 2 minutes. Add the tomatoes and wine, scrape to dislodge any browned bits stuck to the bottom of the pot. Bring to a boil and simmer for 2 minutes. Return the shanks to the pot, add 10 cups of water and bring to a boil. Transfer the casserole to the oven and braise until the meat is very tender, about 2 hours. Let cool slightly.
When the meat is cool enough to handle, pull the meat from the bones and return it to the pot. Discard the bones, cartilage and fat. Cover the stew and refrigerate overnight.
Bring a large pot of salted water to a boil. Add the cabbage and blanch for 1 minute; drain well.
Discard the fat that has solidified on the top of the stew. Rewarm the stew, then strain the broth into another large pot; reserve the meat and vegetables. Bring the broth to a simmer and add the cabbage. Cover and simmer over moderate heat until the cabbage is very tender, about 1 hour. Add the reserved meat and vegetables, season with salt and pepper and remove from the heat.
Preheat the oven to 350°. In a large skillet, melt 2 tablespoons of the butter over moderate heat. Add half of the baguette slices in a single layer and toast until golden brown, about 1 minute per side. Wipe out the skillet and repeat with the remaining 2 tablespoons of butter and the remaining baguette slices.
Line the bottom of a clean casserole or individual ovenproof bowls with half of the baguette toasts. Top with half of the Gruyère and fill with the stew. Cover with the remaining toasts and then with the remaining cheese. Transfer carefully to the oven and bake until the cheese is golden and bubbling, about 20 to 45 minutes. Serve hot.