Melt butter in a medium nonstick skillet over medium heat.
Add onion and garlic; saut 3 minutes.
Add mushrooms and artichoke hearts; saut 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.