1Heat butter in 7- to 10-inch nonstick omelet pan or skillet over medium-high heat until hot. Add mushrooms; saute until tender, 4 to 5 minutes. Add tomatoes; saute until heated through, 1 minute.
2Remove from pan; keep warm.
3Beat eggs and dressing in small bowl until blended. Heat omelet pan over medium-high heat until hot.
4Pour in egg mixture. Mixture should set immediately at edges.
5Gently push cooked portions from edges toward the center with inverted turner so that uncooked eggs can reach the hot pan surface. Continue cooking, tilting pan and gently moving cooked portions as needed.
6When top surface of eggs is thickened and no visible liquid egg remains, place mushroom filling on one side of the omelet; sprinkle with cheese and parsley. Fold omelet in half with turner. With a quick flip of the wrist, turn pan and invert or slide omelet onto plate. Serve immediately.