Preheat oven to 375 Fahrenheit. Prepare a cast iron skillet with a drizzle of olive oil. Prepare a roasting pan for later and set aside.
Cut pork loin in half length-wise. Salt & pepper both sides of the loin generously.
Layer inside of the pork loin with spinach and then cheese.
Carefully roll the pork loin and secure with toothpicks.
Turn cast iron skillet to high heat.
Once oil in skillet is shimmering (not smoking), place pork loin down. Sear on each side for 3 minutes.
Add pork loin to a roasting pan and roast for 15 minutes or until cooked through (usually 140-150 degrees Fahrenheit).
Let pork rest for 10 minutes tented under foil. Discard the toothpicks. Slice pork and serve immediately.