Spain has one of my favorite omelets made from potatoes, cheese and onions called a "tortilla." Not to be confused with a Mexican tortilla, a Spanish tortilla is like the perfect mash-up of a baked potato and scrambled eggs. The Spanish serve it like an appetizer, but I love making it into an omelet. The key is to season every layer with salt and pepper at each stage. Small yellow potatoes are a bit more work to peel and dice, but they're sweeter than larger russet potatoes.
Preheat the oven in broiler mode and place a rack 10 to 12 inches (25 to 30.5 centimeters) below the heating element.
Beat the eggs in a bowl and add 1/2 teaspoon of salt, 1/4 teaspoon of pepper and 1/4 cup (25 grams) of the Parmesan cheese.
Place the peeled and diced potatoes in a microwave-safe bowl, then toss with 2 tablespoons (30 milliliters) of the olive oil and season with 1/4 teaspoon salt. Cover tightly with plastic wrap and microwave on high for 6 to 9 minutes total, or until fork tender.
Meanwhile, heat the remaining 2 tablespoons (30 milliliters) of olive oil and the butter over medium heat in a heavy 10-inch (25-centimeter) omelet pan. Add the onion and a generous pinch of salt. Cook, stirring, until tender but not browned, about 5 minutes. Add the potatoes to the pan and toss gently so that the potatoes don't break apart. Season the mixture with 1/2 teaspoon salt and 1/4 teaspoon pepper.
Pour the reserved egg mixture into the pan over the potatoes and onions. Shake the pan gently while you lift the edges of the frittata; repeat until the eggs start to set, 2 to 3 minutes. Turn the stove to low and sprinkle the mozzarella and remaining 1/4 cup (25 grams) of Parmesan over the top of the frittata.
Once the frittata is mostly set, place the pan under the broiler for a minute or two to lightly brown the cheese. Remove from the heat and let the tortilla rest for at least 5 minutes. Cut into wedges and serve with some sour cream, if desired.
Reprinted with permission from "101 Epic Dishes" by Jet Tila and Ali Tila, Page Street Publishing Co. 2019. Photograph by Ken Goodman.