Heat a large, heavy frying pan over high heat for 2 minutes, or until very hot. Season steaks with salt and pepper to taste.
Place steaks in hot frying pan and immediately turn down the heat to medium high. Cook for 4 minutes. Turn steaks and cook for an additional 4 to 6 minutes, depending on your preference. Remove steaks from frying pan to a platter and cover with foil to keep warm.
Remove frying pan from heat. Quickly cook the garlic for 1 to 2 minutes in the frying pan using residual heat. Return frying pan to medium low heat and add Marsala and chicken stock. Bring to the boil and reduce for 3 minutes.
Remove pan from heat and whisk in butter and rosemary. Serve sauce over steaks.