Melt butter in a frying pan 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 the boil, and cook until the mixture has reduced to about 125ml. Set aside. This may also be made ahead of time, and reheated.
Preheat the oven to 180 C / Gas mark 4. Heat oil in a cast-iron or other oven-safe frying pan 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 - with an internal temperature of 65 C. You may adjust this time to allow the steaks to finish just below your desired degree of doneness if medium is not what you prefer. Remove from the oven, and place on a baking tray. Stir together the breadcrumbs and Stilton. Top each steak with a layer of this mixture.