Preheat the oven to 160°C / 325°F. Trim excess fat off the beef cheeks and season with salt and pepper. In a large heavy pan heat a splash of olive oil and brown the cheeks quickly, set aside.
In a large saucepan, brown the Mirepoix evenly, first the carrots, then the celery, and finally the shallot. Place the meat, Mirepoix, and bouquet garni in a large casserole dish. Add the vegetable broth, covering about 2/3 of the meat. Cover the casserole dish and bake for 2 ½ hours or until tender. Transfer the meat to a platter, cover, and purée the sauce, vegetables, and chocolate in a food processor or blender until velvety and creamy. Season the sauce with salt and pepper to taste.
While the meat is in the oven, prepare the sides:
For the Jerusalem artichoke mousse, in a large saucepan, heat a splash of olive oil and sear the shallot until golden and soft. Add the Jerusalem artichokes and the Marsala and bring it up to a simmer, cover and cook for about 15 minutes or until tender.
Take the pot off the heat and stir in the cream, then purée in a food processor or blender until smooth, season with salt to taste. Let it cool, then pour into a mousse dispenser and cool in the fridge for 1 hour. In case you don’t have a mousse dispenser, set aside and serve the mousse using a spoon.
Preheat the oven to 150°C / 300°F. For the sweet potatoes, season the potatoes with salt and a drizzle of olive oil and wrap in foil. Bake for about 1 hour or until soft. Scrape the soft potatoes out of their skin, discard the skin, and purée in a food processor or blender until creamy; set aside.
For the mushrooms, in a large heavy pan, heat a splash of olive oil over high heat and sear the porcini quickly on both sides for about 30 seconds to 1 minute until golden brown, but not soft; set aside.
Arrange a piece of the beef cheek in the middle of each large plate and glaze with the sauce. Using a spoon or the mousse dispenser, place a dollop of the Jerusalem artichoke mousse and the sweet potato purée next to the meat. Divide the porcini and truffle between the plates, sprinkle with chocolate, and drizzle with a little olive oil. Serve immediately.