Trim fat from tenderloins.
Cut a lengthwise slit down center of each tenderloin about 2/3 of the way through the meat.
Place tenderloins in opposite directions side-by-side on a rack that is coated with a nonstick cooking spray.
Place the rack in a shallow roasting pan.
Coat a large skillet with a nonstick cooking spray.
Add margarine and place over medium heat until hot.
Add onion and saute until tender, stirring frequently.
Add seasonings, stirring well.
Spread onion mixture evenly over tenderloins.
Bake in a preheated 400° Dutch oven for 40 minutes or until done