Season the steak with kosher salt or steak seasoning of your choice. If you have the time let sit refrigerated for the salt to better penetrate the surface. When ready to grill make sure the steak is cool, not at room temperature.

Clean and lightly grease grill grates. Preheat the grill (charcoal or gas) to very hot (about 500 F). If it has a searing side burner/sear box use that instead.

Place the steak over direct heat and cook for 5 minutes on one side. Flip over and cook until the internal temperature of your desired doneness is reached. About 4 minutes, most likely less for medium-rare. (Use a meat thermometer and consult the temperature ranges provided below.)

Rest the flat iron for about 5 minutes before you slice it against the grain. (You may loosely tent it with foil.) Serve with cowboy butter.

Heat a small skillet over medium heat. Melt 2 tablespoons of the butter and saute the shallot, garlic and thyme until soft, 2-3 minutes.

Stir in the mustard and Worcestershire sauce, then add lemon juice and red pepper flakes.

Add the remaining butter (cut in pieces is helpful) and chopped parsley. Once the butter has melted transfer to a bowl and serve the sauce.