Preheat oven to 375 degrees F. Line a 12-cup muffin tin with silicone or spray with nonstick cooking spray.
In a large bowl, whisk together eggs, egg whites, salt, and pepper.
Evenly divide spinach, bell pepper, and onion among the muffin cups.
Pour the egg mixture over the vegetables, filling each cup a little more than 1/2 full.
Spoon small dollops of Boursin cheese on top of each cup.
Bake for 18-22 minutes, or until eggs are set in the center.
Let cool for 5 minutes before removing from the muffin tin.