Stir together dressing and mustard.
Brush 1/4 cup sauce on both sides of fish; reserve remaining sauce.
Arrange fish in 8-inch square baking dish with thickest portions toward outside of dish.
Cover with plastic wrap; vent.
Microwave on high 4 to 7 minutes, turning dish after 3 minutes.
Let stand, covered, 2 to 3 minutes or until fish flakes easily with fork.
Serve with remaining sauce.