Adapted from Deborah Madison's Vegetarian Cooking for Everyone , this frittata is made from start to finish in a cast-iron pan.
• scant 1 ⁄ 3 cup olive oil (fruity Spanish olive oil is best here, but not necessary)
• about 1 ½ pounds potatoes (any type), or as many as you have on hand that will fit into your pan, peeled and very thinly sliced
• 1 medium onion, peeled and sliced
• 5 eggs, beaten
• salt and pepper
• about 2 teaspoons rosemary, minced
Generously film an eight-inch cast-iron skillet with olive oil. *Note: if you don't have cast iron, you can use a non-stick skillet; you will just have to flip the frittata later in order to cook both sides, rather than putting it in the oven to cook. And if you don't have a pan of this size, you can use a larger pan and add more eggs and potatoes.
Add the onions and cook, stirring, until they have stared to color. Now add the potatoes and rosemary and cook over medium heat, stirring frequently, until they are golden and the onions have browned.
Transfer the onions, potatoes, and rosemary to a bowl. Wipe out the pan, add more olive oil, and turn the heat to low. Turn your broiler on, or just heat the oven to a high temperature (around 450 or so) and move the oven rack to the top third of the oven.
Pour the eggs over the potato and onion mixture, then pour the whole thing into the pan. Smooth down any potatoes that stick up, then cook over low heat. When you see the edges of the frittata starting to firm up and the center becoming opaque, test with a fork to see if the egg is cooked. (You want something slightly firm and spongy rather than wet and raw.)
When the bottom of the frittata seems cooked, place it under the broiler or in the oven to cook the top. Keep a careful eye on it so that it doesn't burn; when it's puffy and golden on top, it's done. *If you don't have an oven-proof pan: invert the frittata onto a plate, then slide it back into the pan so that the cooked side is on top, and cook for a few minutes. This is the proper way to cook a frittata, but I find the flipping a little nerve-wracking, and the oven method has always served me well.