Some things you will need before we get started.
1.) A sirloin steak.
2.) Cast iron skillet.
3.) A stove.
6.) Square of tinfoil large enough to completely encase your steak.
First, we prepare our steak. I would recommend lightly sprinkling each side of the steak with salt and pepper and letting it sit for an hour. With about 20 minutes left in that hour, turn the stove top on high and place the skillet on. We will need a very hot skillet.
After the skillet is nice and hot, place the sirloin steak on. There will be a loud hissing sound and that is fine. Once you place it on, be sure to move it vertically ever so slightly so that the steak doesn't stick to the skillet. We want those nice black lines on our steak. To prepare it medium-rare/medium, we are going to cook the steak 2 minutes on each side.
After the first two minutes on the first side, flip it over to the next side. Repeat the process of moving the steak ever so slightly to ensure it doesn't stick. After two minutes, flip it over again. When flipping it over be sure that the steak is set so that the lines are perpendicular to each other. We are aiming for some nice perpendicular lines, forming some delicious black-lined squares. Do another 2 minutes on this side. Flip and repeat the process.
The total amount of time our sirloin has been on the grill is 8 minutes. Now take the steak off and wrap it up in the tinfoil we have prepared. You want to let your sirloin rest for 5 minutes. During these 5 minutes the meat will continue to cook. If it looks a little too rare, wait the 5 minutes out and check it again.
A careful note, the amount of time a steak need on the skillet is dependent on the thickness. 2 minutes a side is the general rule for an inch thick sirloin steak. Some steaks will cook faster due to fat content. I do not recommend cutting a steak open during the cooking process because it may lose some of its juices. I tend to gauge my steaks by their hardness.
A soft steak is rare and a very hard one is well done. A medium steak should have some give, but not too much. As a last measure you can cut your sirloin just to be sure.