Onion-Topped Pork Tenderloin

Sauteed onions give this tender, savory pork tenderloin a delicious rich sweetness. My meat-loving husband said it was very tasty! 1 (2 lb) pork tenderloin cooking spray 1 tablespoon butter or margarine 2 cups sliced onion 1/2 teaspoon dried thyme 1/2 teaspoon salt...