Pre-heat your oven to 400 degrees.
Take a one-pound package of ground sirloin and just cut it in half. One of the secrets to great burgers I've learned is to handle the meat AS LITTLE AS POSSIBLE. Just take your cut-off section and pat the edges til you get it rounded as you please. You are making something that resembles a large hockey puck about one inch thick. Don't squish it down.
Spray the bottom of your cast iron pan with olive oil. Heat it until just starting to smoke.
While it's heating, spray the top of your burger with more olive oil spray and sprinkle with salt and pepper. When the pan is ready, put your burger on a spatula and lower it into the pan. Sizzle time on Medium High. Don't touch the burger. Let it cook two minutes. Turn over. Sizzle two more minutes on the other side. Again, do not be tempted to squish it with your spatula. This makes your burger tough. Then lift the entire cast iron pan (use an oven mitt) and put it in your 400-degree oven for 7 more minutes. The burger will come out between Medium and Medium Well, mostly on the Medium Well side. If you are nervous about how done it is, poke a little slice and take a look. The burger will continue to cook, though, even out of the oven. Let it sit for a minute so it soaks the juices back and then serve with your sides and condiments.
If you do not have a cast iron pan you can transfer your burger to a Pyrex pan that can handle the heat for the roasting part or some pans you can wrap the handle in aluminum foil and it will be okay. I'll trust you to find the right thing for you. I'd hate to see you ruin a pan with a plastic handle.
You can do more than one burger at a time; just give them some breathing room in-between or they give off so much steam they don't cook correctly.