In a large nonstick oven-going skillet, cook onion, covered, in 2 teaspoons of the oil over medium heat for 10 minutes, stirring occasionally. Uncover and add mushrooms. Cook for 5 minutes. Add chopped pear and thyme. Cook for 3 to 5 minutes or until pear is just tender, stirring occasionally. Remove from heat and set aside.
Meanwhile, preheat oven to 400 degrees F. Using a sharp knife, cut a large pocket in the side of the turkey tenderloin by cutting horizontally into the tenderloin to, but not through, the opposite side. Spoon onion mixture into the pocket. Secure opening with wooden toothpicks. Sprinkle top of tenderloin with salt and pepper.
Carefully wipe out the skillet. Add remaining 1 teaspoon oil to the skillet; heat over medium heat. Add stuffed tenderloin, top side down, to hot skillet. Cook for 5 minutes or until browned. Turn tenderloin.
Roast, uncovered, in the oven about 20 minutes or until no longer pink (170 degrees F). Cover with foil and let stand for 5 minutes. Slice to serve.