Preheat the oven to 250 degrees. Brown the short rib in a hot pan with vegetable oil. Transfer to an oven dish, and add the stock, garlic and thyme. Tightly seal the dish with foil and cook for 4 hours. Remove the foil for the last 20 minutes, or until crisped.
Preheat the oven to broil. Butter a baking dish.
Melt the butter in a large saucepan. Add the flour, and cook for one minute while stirring. Gradually add the milk, beating well after each addition to avoid lumps. Add a generous amount of black pepper, the grated Gruyere and half the Parmesan. Cook until the cheese has melted, then turn off the heat and add in the cooked pasta. Pick the meat from the short rib and add to the pan, then mix it all together.
Transfer the macaroni to the baking dish and top with the remaining Parmesan.
Put the breadcrumbs, thyme, parsley, salt and olive oil into a food processor and pulse a few times until mixed through. Sprinkle on top of the pasta. Grill for 5 to 10 minutes until golden brown and bubbling. Leave to rest for a few minutes before shaving the truffle on top, then serve.