Preheat the oven to 160°C.
Heat 60ml of olive oil in a large oven-proof dish.
Brown the beef in batches over moderate to high heat and reserve in a bowl.
Deglaze the dish with stock, scraping the base, and add the liquid to the meat.
Heat the remaining 30ml of oil and soften the carrots, celery, and onions.
Return the meat to the dish and cook with flour for two minutes.
Add enough stock to almost cover the meat and season with salt and pepper.
Stir in the thyme and bay leaf and simmer on low heat.
Grate the potatoes and peeled parsnips using a coarse grater.
Melt the butter with mustard seeds in a separate pan.
Toss the grated vegetables in the butter mixture to coat evenly.
Spread the vegetable crust over the beef stew.
Insert the beef bone upright into the center of the crust.
Cover with foil and bake for 2 hours, removing the foil after 1 hour.