||Pre-heat oven to 400° F (200° C). Shred the potato and cheese, set cheese aside.
||Spray a 9-inch glass or ceramic pie dish with non-stick spray (like Pam). Toss potatoes with 1 teaspoon oil and about 1/8 tsp each salt and pepper.
||Press into an even layer in pie dish, up the sides like a crust. Bake until golden brown at the edges and dry, about 20 minutes. Let cool.
||While the crust is baking, chop the onions and mushrooms, set aside.
||After the crust is done baking, lower the oven to 325° F (160° C). . Heat 1/2 tbsp oil in a large skillet, preferably non-stick, over medium heat. Add onion and cook until softened and golden, about 5 minutes. Add remaining oil and mushrooms and cook, stirring, until mushrooms release their liquid and most of the liquid evaporates, about 8 minutes. Add spinach and 1/4 teaspoon each salt and pepper, and cook, stirring, just until spinach wilts, about 30 seconds. Let cool slightly.
||Whisk together eggs, milk and remaining 1/8 teaspoon salt and pepper. Spread the mushroom mixture in an even layer in the pie dish, and top with an even layer of the cheese. Pour in egg mixture.
||Bake until firm around the edges but still wobbly in the center, about 20 minutes. Let cool, and serve warm or at room temperature.
||Note: you can also substitute other vegetables if you want some variety such as bell peppers and asparagus. Try substituting goat cheese or feta instead of gruyere for a salty kick of flavor. If you are watching the sodium, half the amount of salt, but keep all the pepper, it will still be very flavorful.