Trim fat from pork. Brush Worcestershire sauce over pork, and sprinkle with salt and pepper; set aside.
Combine the onion and garlic in an 8-inch square baking dish; stir well. Cover with wax paper, and microwave at high for 2 minutes. Stir flour and thyme into onion mixture; gradually add broth and mustard, stirring with a wire whisk until blended. Arrange pork on top of onion mixture, spoke-fashion, with ends pointing toward center of dish. Cover and microwave at high 7 minutes, rearranging pork and stirring the onion mixture every 3 minutes. Let stand, covered, 5 minutes.