Place a skillet over medium-high heat and spray with nonstick cooking spray.
Saute onion and peppers until tender.
Add ham and cook for an additional minute.
Remove vegetables and ham from the skillet and set aside.
Wipe out skillet and spray with nonstick cooking spray.
Pour egg whites into the skillet, tilt to cover the bottom of the pan in an even layer and cover with a lid for 3-5 minutes, or until eggs are set.
Remove lid and top half of the omelet with vegetables, ham and cheese.
Use a rubber spatula to lift the edges of the omelet and fold in half.
Carefully slide the omelet onto a plate and serve.