Butterfly your tenderloin carefully using a sharp knife.
Cut horizontally but don't slice all the way through.
Open it like a book to create flatness.
Pound it gently to even out the thickness.
Season both sides with salt and pepper generously.
Sauté spinach in olive oil until it wilts down.
Add minced garlic and cook for one minute more.
Mix in chopped sun-dried tomatoes and feta cheese.
Let this mixture cool down before using it.
Spread the stuffing evenly over the flattened beef.
Leave a one-inch border around all the edges.
Roll the tenderloin tightly from one long side.
Tie it securely with kitchen twine every two inches.
Heat olive oil in your roasting pan first.
Sear all sides until they turn golden brown.
This locks in all the delicious juices inside.
It takes about two minutes per side total.
Place the pan in a preheated 400°F oven.
Roast for 25 to 30 minutes for medium-rare.
Use your meat thermometer to check internal temperature.
Aim for 135°F for perfect medium-rare doneness.
Remove from oven and cover with aluminum foil.
Let it rest for 10 minutes before slicing.
This keeps all the juices inside the meat.
Slice into beautiful rounds and serve immediately to guests.