2 tablespoons olive oil, divided
2 medium leeks, whites and pale green parts only, chopped
8 ounces crimini (baby bella) mushrooms , thinly sliced
12 large eggs
1/2 cup crème fraîche or sour cream
2 tablespoons coarsely chopped flat-leaf parsley
3/4 cup shredded Fontina cheese, divided
Kosher salt, freshly ground pepper
Place a rack in upper third of oven; preheat to 350°.
Heat 1 tablespoon oil in a 10-inch nonstick ovenproof skillet over medium heat. Add leeks; cook, stirring often, until softened, about 5 minutes. Add mushrooms and cook, stirring often, until softened and all liquid has evaporated, 8-10 minutes.
Meanwhile, whisk eggs, crème fraîche, and parsley in a large bowl; mix in 1/2 cup cheese. Season with salt and pepper.
Increase the heat to medium-high and add remaining 1 tablespoon oil to the skillet. Pour the egg mixture over the mushrooms, shaking the pan to evenly distribute mixture.
Cook the frittata, without stirring, until its edges begin to set, about 5 minutes.
Sprinkle remaining 1/4 cup cheese over eggs and transfer skillet to oven. Bake frittata until golden brown and center is set, 25-30 minutes.