Melt butter in a frypan over medium heat. Add the onion, garlic and thyme. Cook, stirring constantly, until onion is tender. Stir in the beef stock, scraping any onion bits from the bottom of the pan, then stir in the port wine. Bring to a boil and cook until the mixture has reduced to about 1/2 cup. Set aside. This may also be made ahead of time and reheated.
Preheat the oven to 180C. Heat oil in a cast-iron or other oven-safe frypan over high heat. Sear steaks quickly on both sides until brown then place the whole pan into the oven.
Roast steaks in the oven for about 15 minutes for medium rare. Remove from the oven and place on a baking tray. Stir together the panko crumbs and blue cheese. Top each steak with a layer of this mixture.
Preheat the grill. Place steaks under the grill until the cheese topping is browned and bubbly, 3 to 4 minutes. Remove from the oven and let stand for at least 15 minutes before serving. Serve with warm port wine sauce.