Place the pork tenderloin in a 24-oz shallow casserole. Sprinkle with salt and onion. Cover with vented plastic wrap and microwave, on medium (50%) power for 4 minutes; rotate the casserole 1/4 turn. Microwave 4 minutes longer. Mix the remaining ingredients; spoon over the pork. Cover with vented plastic wrap and microwave, turning the casserole 1/4 turn every 3 minutes, until pork is done, 4 to 6 minutes.