Cast iron utensils have been American favorites since colonial times. Although iron does not transfer heat as capably as aluminum or copper, it offers some practical advantages. Cast iron is slow to heat up, compared to other metals, but once it arrives at a working temperature it retains its heat superbly. That makes it an excellent material for skillets and griddles, which are often preheated before they are used. Cast iron remains at a searing temperature longer than other metals, making it ideal for quick-cooking meats like steaks, chops or a pork tenderloin.
Trim any large pieces of surface fat from the tenderloins. Most tenderloins have a partial sheath of tough connective tissue, called "silverskin," on top. Remove this by sliding the tip of a sharp knife underneath the layer, and sliding the knife to the end of the tenderloin. Angle the blade upward, against the silverskin, to avoid unnecessary waste.
Season the tenderloin lightly with salt and pepper. If desired, rub the tenderloins with a dry seasoning mixture, or immerse in a marinade. Cover and refrigerate for three to four hours, or as directed in your favorite recipe.
Remove the tenderloins from the refrigerator, if applicable, and wipe away any excess marinade. Pat the surfaces dry with clean paper towels. Cut the tenderloins in half across the width, if necessary, so they will fit in the skillet. Use two skillets if necessary.
Oil the skillet lightly, and place it over a medium-hot burner. When the oil is shimmering and almost hot enough to smoke, place the tenderloins in the skillet. Sear them carefully on all sides, until well browned.
Reduce heat, and continue to cook the tenderloins, turning regularly, until the internal temperature of the tenderloin reaches 160 degrees Fahrenheit in the thickest part of the loin. Use a probe thermometer or a calibrated instant-read thermometer for accuracy.
Cover the tenderloins and let them rest for five or ten minutes. Serve with your favorite side dishes.