Sauté onion in hot oil in a large skillet over medium heat until tender. Add spinach, salt, and pepper, and sauté 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 at 425° for 50 minutes or until a meat thermometer inserted into thickest portion of tenderloin registers 145° (medium-rare). Let stand 10 minutes before slicing; garnish, if desired.