In a heavy saucepan over high heat combine wine, chicken stock and beef stock. Bring to the boil and cook until mixture is reduced to 480ml, about an hour. Season with salt to taste. (This can be prepared a day ahead - cover and refrigerate).
In a small bowl, mix butter and flour. Heat 1 tablespoon olive oil in a large frying pan over medium heat. Sprinkle steaks with salt and pepper, and cook until medium-rare, about 4 minutes per side. Transfer steaks to a plate.
Stir shallots, garlic and thyme into frying pan. Add 480ml reduced wine mixture to frying pan. Bring mixture to the boil, scraping up any browned bits. Whisk in butter mixture until smooth. Boil sauce until thick enough to coat a spoon, about 2 to 3 minutes.