Caprese Frittata

Ingredients

1 tablespoon olive oil
½ cup coarsely chopped yellow onion
4 ounces spinach, chopped
8 eggs (8 whites and 4 yolks), lightly beaten
6 cherry or grape tomatoes, halved
2 ounces fresh mozzarella, shredded or chopped
2 tablespoons torn basil

Preparation method