As soon as you can (up to 4 days in advance, and at least a few hours before), salt the steaks by covering all sides with a good amount of kosher salt, but not too much. (Some people say to rinse off the salt before cooking but that sounds like a bad idea to me.) Place in the fridge.
About 90 minutes before you're ready to cook the steaks, remove them from the fridge and allow to warm up a bit. This is, according to some science-based food writers, apocryphal, because the internal temperature increases very slowly so the effect is minimal. I still recommend it.
When ready to cook, put the cast-iron on the stove and heat to medium-high. Add canola oil as the pan warms up. Heat for a minute or two, but not too long. Oil should be shimmering but not smoking heavily.
Add the steaks carefully. Flip frequently (every 15 seconds) for up to 4 minutes, until a basic crust begins to form. Turn the heat to medium to start allowing the pan to cool a tiny bit.
Then, add some butter to the pan along with the thyme and shallots. Continue flipping, though less frequently, and use a spoon to baste the meet. If you are seeing lots of smoke, turn the heat further down.
Start checking the temperature after a minute or two of the basting. Remember, the steak will continue to cook off the pan. For medium-rare meat, a final temperature of 130ºF (or 125ºF in the pan) is ideal; for medium, (or 135ºF in the pan) 140ºF is your goal.
Place the steak on a plate or cutting board Let rest for a few minutes.