Preheat the oven to 375 degrees Fahrenheit, with the middle rack in position.
To make the frittata: Heat some olive oil over medium heat in Large Fry. Sauté the onions first, with a little salt, until they start to soften. Then add the speck (optional) and the mushrooms. Season with salt and ground white pepper to taste. Turn off the heat when the mushrooms have cooked and onions have fully softened.
In a large mixing bowl, whisk the eggs until fluffy. Add the Fontina, a sprinkle of salt, and white pepper, and whisk again. Lightly oil your Hot Dish, spread the onion mixture evenly in the pan, and add your whisked eggs. Bake in the oven for 20 to 30 minutes, until fluffy and cooked through (no jiggle in the middle when you shake it!).
Make the rösti while the frittata bakes. Peel and shred the potatoes with a cheese grater. Sprinkle with salt, mix, and place in a strainer until you are ready to use. You’ll want to work quickly so the potatoes don’t brown.
Heat a generous swirl of olive oil in Large Fry, turn the heat to medium, and add your shredded potatoes in a thin layer across the bottom of the pan. Press down with a spatula until the potatoes form a large, compact circle.
The thicker your rösti is, the less crispy it will be — if you have more potato mixture than a thin layer calls for, set it aside and make another one afterward instead of making a thicker rösti. When the bottom side is golden brown, flip it over with your spatula and cook until crispy and potatoes are cooked through.
Remove the rösti from the pan and either serve it on the side or cut it up and place it on top of the frittata. Garnish the dish with fresh herbs like chives, chervil, and parsley.
Photos by Farrah Skeiky.