Leek, Mushroom and Ricotta Cheese Frittata

Heat oven to 400 °F. Heat the oil in a 10-in. nonstick oven-safe skillet over medium heat. Add the leeks and 1/2 tsp each salt and pepper and cook, covered, stirring occasionally, for 5 minutes. Increase the heat to medium-high, add the mushrooms and cook, tossing occasionally, until golden brown and tender, 4 to 5 minutes. Meanwhile, in a large bowl, whisk together the eggs, ricotta and Parmesan. Add the egg mixture to the skillet and stir to distribute the ingredients. Transfer to the oven and bake until a knife inserted in the center comes out clean, 16 to 18 minutes. Transfer to a cutting board. Serve with a salad, if desired.