I described the French definition of mignon above. The filet mignon is a slice, or section, of the tenderloin, or Chateaubriand, a long piece of choice meat that rides along the protected inside waist of a cow. It is the tenderest of all cuts. If you like filet mignon, and if you want a lot of it, you order the Chateaubriand for two. That way they bring you the whole tenderloin on a big platter, surounded by various veggies and whatnot. You can slice it as thin or thck as you want. Then you and your best pard or SO dig in. Or, if you are like me after a few weeks at sea, you freak the wait staff out by eating the whole platter yourself with a bottle of heavy, full bodied red, like a Sangiovese, or a run of the mill Chateauneuf du Pape. LOL. No dessert cart, please.
I’m not sure what a KC cut is. I spent very little time in the Midwest. But, from what you describe, it sounds like what we on the east coast call a NY cut. A NY cut is the beef found on the opposite side of the bone from the tenderloin. It has more texture and taste than the filet. The Porterhouse, which includes both cuts, is taken from the top of the shortloin of the cow. There isn’t much top shortloin to each cow, so it is an expensive cut. The NY strip is simply a T-bone without the bone.
There is also a benefit to cooking the meat on the bone that many people aren’t aware of: I tastes a lot better. It has a fuller beefy taste. Also, because the bone holds heat like a rock, your steak will stay warm longer.
If you’re not a big eater, order the Porterhouse anyway. That filet will taste great at breakfast the following day with eggs and lyonnaise potatoes. For the price you pay for the fine cuts of steak, in a restaurant or at the butcher’s, the Porterhouse is the best buy.