3 ounces cremini or button mushrooms, trimmed and sliced (about 1 cup)
1 small (about 3 ounces) russet potato, cut into 1/4- inch cubes (about 1/2 cup)
1/2 red bell pepper, cut into 1/4- inch cubes (about 3/4 cup)
4 large eggs
1/2 cup crumbled low- fat feta cheese
1 teaspoon chopped fresh oregano leaves
1/4 teaspoon ground black pepper
Set a rack about 4 inches from the broiler and preheat the broiler to low.
Heat an 8- to 10- inch cast- iron or other ovenproof (not nonstick) skillet over low heat. Add the spinach with the rinse water still clinging to the leaves (if the spinach is completely dry, add a tablespoon of water to the pan with the spinach). Cover and cook until the leaves have barely wilted, about 1 minute. Transfer to a colander and use tongs to squeeze out as much liquid as possible. Wipe the skillet clean.
Add the olive oil to the cleaned skillet and heat over medium heat. Add the onion and salt and cook, stirring occasionally, until the onion is beginning to soften, about 2 minutes. Add the mushrooms and cook, stirring occasionally, until lightly browned, about 2 minutes. Stir in the potato and pepper, cover, and cook, stirring once or twice, until the potato is tender but still firm, another 5 to 7 minutes.
Meanwhile, in a medium bowl, whisk the eggs until lightly beaten. Stir in the reserved spinach, the feta, oregano, and black pepper. When the potato mixture is cooked, evenly pour the egg mixture over it. Cook until the eggs are set around the edges, about 10 minutes.
Place the skillet under the broiler for 1 to 2 minutes, until the top of the frittata is puffed and lightly browned. Serve hot.