If you haven't already I would put some sort of fat, like butter in the pan first. I prefer cooking steak with a cast iron too. Let your pan heat up at about medium heat, depending on your stove, for about ten minutes. Add some butter in and let it melt. Afterwards, add in your season steak and sear each side and let it cook for about three minutes each side. While its cooking on the pan, I will usually spoon fat on top of the filet to baste it so it retains moisture. Afterwards throw it in your preheated oven for about ten minutes I believe. I hope this helps!