The Gagne’s prime rib roast ready for carving. (Photo ©Texas BBQ Posse) Warning: This isn’t a normal Posse blog post. Just how atypical? I had to look up how to spell sous vide. But don’t worry, that vacuum-sealed cooking method was used only for warming up some wonderful leftovers from the meal that long-time Posse member Mike Gagne cooked on Christmas Day. Mike is a University of Texas law school grad and serious Longhorns fan, so when we (Mike is married to my daughter, Libby) decided to cook prime rib for the holiday, he went right to the source of…