Frittata with Leftover Greens

Set an oven rack in the upper third of the oven and preheat the oven's broiler.

Beat eggs and egg whites together in a mixing bowl until smooth. Whisk in parsley, 1/4 teaspoon salt, and 1/4 teaspoon black pepper until evenly blended; set aside.

Heat olive oil in a 10-inch cast-iron skillet over medium heat. Add onion, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon black pepper; cook and stir until onion softens and turns translucent, about 5 minutes. Stir in tomatoes; cook 1 minute. Add cooked Swiss chard; cook and stir until heated through. Pour in egg mixture, cover, and cook until edges of eggs are set, about 5 minutes. Uncover; sprinkle with Parmesan cheese.

Cook in the preheated broiler until cheese is golden brown and center of frittata has set, about 2 minutes. Let stand before serving, 3 minutes.