Roughly chop the celery, onion, carrot, garlic and sauté until browned. Mean while tighten the beef with the string, brush with mustard sprinkle with thyme and using the same vegetable pan seal the beef from all sides. Place the vegetables on an oven tray with the beef on top and roast in the oven until cooked to your likings. Once the beef is removed from the pan pour in the port and wine, reduce by half than add the cream to simmer again for a few minutes. Trim the beans, slice the oyster mushrooms and sauté in garlic butter. Slice the potatoes as thin as possible, melt the cheese with the cream and mix with the potatoes, mix in 3 tbsp grated cheese and place in an oven tray. Sprinkle the left cheese on top and roast in the oven until potato is cook, remove from heat and allow to rest before portioning.