1 tablespoon extra-virgin olive oil 2 onions, halved and very thinly sliced 1/2 teaspoon fine sea salt 4 cups loosely packed fresh arugula, stemmed 8 eggs 2 cups tomato sauce
Preheat the oven to 425 degrees. Butter a 9-inch springform pan and set it aside.
In a large skillet, combine the oil, onions and salt. Sweat (cook, covered, over low heat until soft but not browned) for about 5 minutes. Add the arugula; cook just until wilted, about 1 minute, and remove from the heat. Set aside.
In a large bowl, gently beat the eggs with a fork. Add the onion-arugula mixture and mix to blend. Pour the mixture into the springform pan. Place the pan in the center of the oven and bake until firm and golden, 35 to 40 minutes.
Transfer the pan to a rack and allow to cool for 10 minutes. Then run a knife around the sides of the pan; release and remove the springform side, leaving the omelet on the pan base. Serve at room temperature, cut into wedges, accompanied by tomato sauce.