You sear both sides for 30-45 seconds and then let the steak rest for about 2 minutes, and then repeat until you reach the internal temp you want. Usually about 3 or 4 times for a reasonably thick steak.
I can highly recommend this method and it's also good for cooking large numbers of steaks because you can do 2 or even 3 batches at the same time with the same cast iron pan.
Just make sure to save those juices that drip down for your pan sauce if you are making one!
The run included mustard as a base with lots of the predictable spices.
Put a cast iron skillet (which holds lots of heat) in oven at 500F on the top rack. Once it's hot, add the steak and turn on the broiler.
Flip 1/2 way through. Takes 6-12 mins. You can get a well-seared exterior with lots of crispy, browned fat, and the inside is still pink.
Tastes great! (Very smoky.)
I preheat 250-300 and watch a remote meat thermometer until the steak is 95% to desired temp. Usually takes 30-45 minutes. Then I crank up a skillet with ghee until it just starts smoking (~500F) and sear the hell out of the steaks for only a minute on each side. Perfect crust every time. Let rest for about 10 minutes.
Personally, I like using butter or ghee. Aside from the taste, it increases and distributes the heat transfer more evenly across the surface which provides a more uniform sear. If you do use oil, I'd recommend using ghee or refined olive oil and make sure to "temper" it first (allow it to warm with the skillet).
I do it just because I don't have to worry about doneness later - the inside is already cooked, just sear it and it's done.