1. Preheat oven to 400°.
2. Trim fat from pork; cut a lengthwise slit down center of each tenderloin, cutting about two-thirds of the way through meat. Place on a broiler pan coated with cooking spray.
3. Melt butter in a large nonstick skillet over medium-high heat. Add onion, and sauté 5 to 6 minutes or until tender. Add thyme, salt, and pepper. Spread onion mixture evenly over tenderloins. Bake at 400° for 45 minutes or until a meat thermometer registers 160° (slightly pink).
carbo rating: 3