Preheat the oven to 200 degrees F.
Season the steaks with salt and pepper. Place the oil in a skillet over medium-high heat. Pan-sear steaks to the desired doneness, allowing 2 to 3 minutes per side for rare; 3 to 4 for medium rare; and 5 minutes for medium. Remove to a plate and keep warm in the oven.
Add the mushrooms, garlic and rosemary to the skillet and cook until the mushrooms are tender, about 5 minutes. Add the cream, salt and pepper and simmer until the cream lightly thickens. Plate the steaks, top with the mushrooms and serve.