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.
There are 249 calories in 1 serving of Mushrooms and Spinach Quiche.