Heat butter in 9-inch pie plate at HIGH 1/2 to 3/4 minute, or until melted, turn plate to coat bottom with butter.
Meanwhile, combine remaining ingredients; pour into pie plate.
Cook, covered with plastic wrap, at MEDIUM 2 to 3 minutes, or until omelet is almost set; stir after 1 minute.
Let stand, covered, 2 minutes.
With spatula, loosen edges of omelet from plate; fold into thirds to serve.