2 tablespoons (1/4 stick) butter
1 cup chopped leeks (white and pale green parts only)
½ lb fresh spinach, rinsed, dried and torn into roughly 1-inch pieces
1 cup sliced stemmed mushrooms
8 large eggs
1 cup diced Fontina cheese, divided
1/2 teaspoon salt
1/2 teaspoon ground black pepper
1/4 cup grated Parmesan cheese
Melt butter in heavy broilerproof 10-inch-diameter nonstick skillet over medium heat. Add leeks and sauté 4 minutes. Add spinach and mushrooms, sprinkle lightly with salt, and sauté until tender, about 6 minutes.
In a medium bowl, Whisk eggs, 3/4 cup Fontina cheese, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Add egg mixture to skillet; fold gently to combine. Cook until almost set. Sprinkle remaining 1/4 cup Fontina cheese and Parmesan cheese over. Broil until frittata is puffed and cheese begins to turn golden, about 3 minutes. Cut into wedges and serve.