The most delicious cut you could cook for your steak-loving valentine is cook them a BONE-IN RIBEYE.
Many people prefer FILET MIGNON for its elegance. It's less fatty but still tender and delicious.
If you're looking for something that's less expensive but still has great flavor, go for a TOP SIRLOIN STEAK.
The best grade of steak is PRIME, followed by CHOICE, then SELECT.
Prime beef is the absolute best. It's produced from young, well-fed cows, and has lots of marbling (fat). Only 2% of the beef in the U.S. is labeled Prime, and most of that goes to restaurants. If you can get your hands on a Prime steak (they'll occasionally carry it at higher-end grocery stores like Whole Foods, or you can order it online here) and are willing to pay a little bit extra, go for it. Otherwise...
Choice beef is widely available and still pretty high quality. Just make sure to look for the steak with the most marbling (fat), since this is what adds flavor and keeps the steak tender during cooking.
Select beef is leaner and typically not as juicy and flavorful. It's also available in supermarkets, and it's an OK choice. But, this is Valentine's Day we're talking about. If cost is a big issue and you go for Select beef, make sure to cook with plenty of fat and baste with butter at the end.
(Yes, there are lower grades of meat, but these are ground, chopped, or used for pet food.)
Once you've procured your steak of choice, you will need kosher salt, freshly ground black pepper, oil/fat* (for cooking), a bunch of fresh thyme, and butter (to finish)...
...plus a 12-inch cast iron skillet*, tongs, a meat thermometer**, a spoon, a cutting board, and a very sharp chef's knife.
Forty-five minutes before cooking, take your steak out of the fridge and dry it thoroughly with paper towels.
Then season it with LOTS of kosher salt, and some pepper.
Let your seasoned steak sit out at room temperature for 45 minutes.
Before you start the actual cooking, open all of your windows, turn your hood fan on, and figure out a way to keep your smoke alarm from going off (this is not a joke).
Although the BEST, EASIEST way to do this is to just put a plastic bag over your smoke alarm and secure it with a rubber band, that is probably illegal and I cannot TECHNICALLY tell you to do it.
BUT: If you do choose to go the "plastic-bag-rubber-band" route, make sure you take the bag off IMMEDIATELY AFTER COOKING. Smoke alarms are no joke.
Also, make sure your windows are all the way open and your hood fan is on. If you have a standing fan, you can point it up toward your smoke alarm to help keep the smoke away from it. If the alarm does go off, fan it with newspaper until it stops.
I wish I was kidding about this, but properly searing a steak results in a lot of smoke. If you've ever had a great steak at a steakhouse, know that there was a LOT of heat and a LOT of smoke involved — and your job is to get as close as possible to that heat and smoke in a home kitchen.
When you're ready to cook (about 20 minutes before you're ready to eat), put your cast iron skillet on a burner over high heat.
Dry your steak again with paper towels.
Add about 2 tablespoons of cooking oil to the pan.
Get it ripping hot before you even think about putting the steak in. Your pan is hot enough when the oil starts to just barely smoke. Use tongs to place your steak(s) in the pan, working away from yourself.
Keeping the heat super high, let your steak cook for 2 minutes without touching it.
After 2 minutes, flip your steak and start to brown the other side.
Continue to flip every 2 minutes until your steak reaches an internal temperature of 120°F (rare). A 1½-inch-thick ribeye steak will get to 120°F in about 8 minutes.
A 1-inch-thick top sirloin steak will get to 120°F in about 6 minutes.
If you're cooking a filet mignon, there are more than 2 sides (the top, the bottom, and the edges), so it's a little more complicated.
To check the temperature of your steak, stick a meat thermometer in, diagonally, so that the tip of the thermometer is in the middle of the steak.
When your steak is at 120°F, it's time to start basting.
To baste: Turn your heat down to medium, and add 3 tablespoons of unsalted butter to the pan.
Let the butter melt and just barely start to bubble, then add about 5 sprigs of thyme.
Then, baste the steak by spooning the melted butter over the top of it, repeatedly.
After a minute, flip the steak and keep basting.
Check the temperature of your steak again. It's done when it's at 130°F (the low end of medium-rare).
Now put your steak on the cutting board and let it rest for 10 minutes.
After 10 minutes, your steak is ready to be sliced. If you cooked a ribeye, the first thing you need to do is cut the meat off of the bone.
Slice your steak against the grain. Your blade should be perpendicular to the wrinkly lines on the meat.
Cut slices half an inch thick, using a single downward stroke for each slice.
Transfer your meat to a serving platter or plate.
Arrange the slices on a serving platter, making sure to fan them out so that your valentine can appreciate the beauty of the perfect medium-rare.
If desired, pour 1 to 2 tablespoons of the fat from the pan over the sliced steak.
Serve with any of the following sides, and your valentine is guaranteed to fall deeply, deeply in love with you.