Melt butter in a medium nonstick skillet over medium heat. Add onion and garlic; sauté 3 minutes. Add mushrooms and artichoke hearts; sauté 2 minutes or until vegetables are tender. Remove from heat, and keep warm.
. Combine egg whites and next 4 ingredients; stir well with a whisk. Heat a small nonstick skillet coated with cooking spray over medium heat. Pour one-third of egg mixture into pan. Cook 3 minutes; turn omelet over. Spread one-third of artichoke mixture onto half of omelet. Sprinkle 1/4 cup cheese over omelet. Carefully loosen omelet with a spatula; fold in half. Cook 1 minute. Slide onto plate, and keep warm. Repeat procedure twice with cooking spray and remaining ingredients.