To make the garnish: Heat the oil in a nonstick skillet. When it is hot, add the mushrooms, salt and pepper. Cook for about 1 minute over high heat. Add the tomatoes and chives and sauté for 10 seconds. Set aside.
To make the omelet: Place the egg whites and salt in a large stainless steel bowl and beat with a whisk until fluffy (about 1 minute), but not at the soft-peak stage. Add the egg yolks and chives and mix them in well.
Heat the butter and oil in a 10" nonstick skillet. When they are hot, add the egg mixture. Cook over high heat, stirring with a fork and shaking the pan at the same time, until the mixture is set (45 seconds to 1 minute). Run a spatula or the tines of the fork around the edge of the omelet to loosen it. Arrange half the mushroom mixture in the center of the omelet. Roll the top and bottom edges toward the center to enclose the filling. Invert the omelet onto a 5- to 6-cup oval gratin dish. Decorate at both ends with the remaining mushrooms and tomatoes.
Serve immediately, cutting the omelet into three pieces.
Reprinted with permission from the French Women Don’t Get Fat Diet click for book review
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