First, if you don't know what kefir is, I'll node it soon, as the only kefir writeup isn't a factual. Suffice it to say that kefir is a fermented milk product similar to yoghurt, but liquid. It also contains oodles of enzymes, including some luvverly meat tenderizers. Besides, it tastes yummy!
So, to start, you take the steaks and thaw them out. (I cheated here, and thawed the outer 3/16" in running hot water so I could get started, and it turned out fine. You'll get more flavour if you thaw them overnight in the refrigerator, though.) Now you have two giant slabs of meat (don't you?) which are ready for some lovin' rub action.
Now sprinkle various and sundry amounts of spices onto each steak. We want onions, red pepper (c'mon, more red pepper than that, ya sissy!), mustard, a little bit of ground coriander, just a bit of cinnamon (trust me, you do want the cinnamon), even some whole black peppercorns. Don't worry too much about using too many peppercorns: they're coming off before it's cooked. Top it off with a liberal sprinkling of salt (plain, or your favourite seasoned salt).
Now press all that stuff into the surface of the steak. You want it to really stick. No, you won't "bruise" the steak by pressing too hard. Just do it already!
Hokay, now do it to the other side, too.
And to the edges. (You did get thick enough steak to warrant doing the edges, didn't you?) The easiest way to do the edges is to sprinkle the spices in a line on the bottom of the pan and press the edges of the steak down onto them.
Finally, the kefir. Lay the steaks in a pan you can seal (such as a casserole dish) completely flat. This isn't a submerged marinade, so they can't be all piled on top of each other.
Pour a quarter cup of kefir over each steak, using a soft basting brush to spread it evenly over the surface and the edges, but not disturbing the rub. Flip the steaks, and cover that side with the drippings of kefir that flowed off the first side. What you're going for is a complete coating of meat-tenderizing goodness.
Now cover and refrigerate overnight. Or longer if you can bring yourself to wait. The kefir will thicken up and form a sort of gel (aided by the salt, for physical-chemical reasons I can't presume to understand). So you'll want to flip the steaks after six or eight hours, gather up the now-gelled drippings of kefir with your brush, and slather them on the exposed side. Do this again after another few hours.
When you really cannot wait any longer, fire up the grill (or, if it's raining for you like it was for me, the oven broiler), take out the steaks, and get ready.
First, scrape all that wonderful kefir-spice-rub goodness off of the steaks. Yes, you heard right. Unless you want your steaks covered in burnt cheese. Don't worry, kefir takes up flavours almost as well as oil, so the steak will be more than permeated with all that is good from the rub.
If you're broiling, pre-heat the oven to 350F (unless you happen to like dry steaks — then you don't have to pre-heat). Switch it to "broil" only when you put the steaks in. Depending on the thickness of your steaks, broiling time will vary. For one-inch steaks, "well done" (aka "You did what to my steak?!?!) is something like seven minutes per side. Do yourself a favour and broil for two or three minutes per side. I mean, honestly! Who wants a steak that can't moo at you anyway?
If you're grilling, I'll not presume to tell you how to do your steaks.
Last step: EAT!
Oh, and it makes great fajita meat, sliced thin...