- 2 medium onions, thinly sliced
- 3 tablespoons olive oil
- 1 large bunch of arugula, tough stems removed, torn into bite-size pieces (about 2 cups)
- 8 large eggs
- ¼ cup freshly grated Parmigiano-Reggiano
- Salt and freshly ground black pepper
- Place the onions in a bowl with cold water to cover. Let stand 1 hour, changing the water once or twice, until the onions taste sweet. Drain and pat dry.
- Pour the oil into a 9-inch nonstick skillet. Add the onions. Cook over medium heat, stirring occasionally, until the onions are tender and golden, about 10 minutes. Stir in the arugula until wilted, about 1 minute.
- In a medium bowl, beat together the eggs, cheese, and salt and pepper to taste. Pour the eggs over the vegetables in the skillet and lower the heat. Cover and cook until the eggs are just set but still moist in the center and the frittata is lightly browned on the bottom, about 5 to 10 minutes.
- Using a spatula to help, slide the frittata onto a plate. Invert the skillet over the plate and quickly flip both the plate and skillet so that the frittata is back in the pan with the cooked side up. Cook until just set in the center, about 5 minutes more. Or, if you prefer not to flip it, slide the skillet under the broiler 3 to 5 minutes or until the eggs are set to taste.
- Slide the frittata onto a serving dish and cut into wedges. Serve hot or at room temperature.