Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add mushrooms to pan; sauté 6 minutes or until moisture evaporates and mushrooms begin to brown. Remove from heat.
Combine 1 tablespoon milk, sour cream, cream cheese, 1/8 teaspoon salt, and pepper in a medium bowl; stir well with a whisk. Add mushrooms to milk mixture; stir well. Combine remaining 2 tablespoons milk, remaining 1/8 teaspoon salt, onions, chives, parsley, tarragon, egg whites, and eggs in a bowl; stir well.
Heat pan over medium-high heat. Coat pan with cooking spray. Spread egg mixture evenly in pan; cook 3 minutes or until center is set. Top with mushroom mixture. Carefully loosen omelet with a spatula; fold in half. Cut omelet in half.