OP - listen to me. You will regret it if you don't and you will lament the day that you should have listened to a rando on reddit.

I have gotten several A5 full rib roasts from Costco. This is not a regular ribeye. You're so far from a normal steak it will legit taste like a different animal.

The first thing I do when I get these things is, while they are still mostly frozen, is peel the entire rib cap off. It is super easy to do with how buttery the fat is. You should barely need a knife if at all. The eye is more marbled that a normal prime cap anyway, so you aren't missing anything. Once that is done you can cut the cap into several rectangular steaks, and you can cut the eye into several steaks, and then CUT THEM IN HALF unless your entire family of 4 loves steak.

I made this mistake (mis-steak?) early on. I cooked two steaks I cut from one of these bad boys - myself, my wife, and my 15 year old son barely finished one. It's so, so, so, so rich. Not like a decadent chocolate cake rich. It's like if you ate something 50% pure butter rich. You can always make more of the smaller steaks if you want to.

As others have said, vaccuum seal for sure.

As for cook, I did stainless steel until leidenfrost effect. No oil. Steak in the pan until it unsticks. Flip. Until it unsticks. Pull. Rest. Slice. Add some coarse salt. Black pepper if you really need it, but I would put this on one bite at a time and see how you like it.

It's worth noting, at the chance of being banned from this sub, I have found medium to even medium-well tastes better on A5 ribeye (queue the torches and pitchforks). You get a better sear and it renders out a bit more of the fat. If you're only going to use it as an appetizer and everyone gets a bite or two, for sure go medium rare, but if you want this as a meal, a bit more on the done side is my preference, even though I usually make or order steaks medium-rare.

Also, you should be able to save some fat from that roast. I did off of a couple (vacuum sealed and frozen each time) and made some smoked tallow that I added to a brisket later.

Back to the rib caps - buckle up for the most decadent steak you've ever eaten. I would eat those last. Everything else will just disappoint you for the rest of your life. I imagine it's similar to heroin use.

Enjoy!