Preheat the oven to 375°. Coat the bottom of a 12- to 14-inch cast-iron skillet with the 1/4 cup of olive oil. Add the onion and marjoram and cook over moderate heat until the onion is translucent and very aromatic, about 7 minutes.
In a medium bowl, whisk the eggs with the ricotta and Parmigiano-Reggiano and season with salt and pepper. Pour the egg mixture into the skillet and stir to incorporate the onion. Cook until the eggs begin to set. Transfer the skillet to the oven and bake for 12 to 15 minutes, until the frittata is set.
Turn the frittata out onto a serving platter and let cool. Drizzle the frittata with olive oil and serve at room temperature, with the tomato sauce on the side.