- 1 onion, thinly sliced
- 1 tablespoon brown sugar
- 2 drops white wine vinegar
- cooking spray
- 4 egg whites
- 2 eggs
- 1/4 cup shredded Gouda cheese
- 1 slice thick-cut prosciutto, cubed
- Cook and stir onion, brown sugar, and vinegar in a skillet over medium heat until onion is golden, about 20 minutes.
- Preheat oven to 350 degrees F (175 degrees C). Spray a 6-muffin tin with cooking spray.
- Whisk egg whites and eggs in a large measuring cup.
- Fill each muffin cup halfway with egg mixture. Scatter onion, Gouda cheese, and prosciutto cubes on top. Cover with remaining egg mixture, leaving 1/4-inch space at the top for eggs to rise.
- Bake in the preheated oven until golden brown on top, about 20 minutes.