Saute onion in hot oil in a large skillet over medium heat until tender.
Add spinach, salt and pepper; saute 1 minute or until spinach wilts.
Remove from heat, and stir in cheese and dried tomatoes.
Cut tenderloin lengthwise down center, cutting to, but not through, bottom.
Lay flat, and spoon spinach mixture down center of tenderloin.
Fold tenderloin over mixture, and tie with string at 1-inch intervals.
(Tenderloin won't completely close).
Chill 2 hours, if desired.
Place cut side up, on a rack in a roasting pan.
Cover exposed filling with a strip of aluminum foil.
Bake tenderloin at 425 degrees for 50 minutes or until the meat thermometer registers 145 degrees (medium-rare).
Let stand 10 minutes before slicing and garnish.
Ready in 1 hour.