Preheat oven to 400 degrees Fahrenheit.

Spray a 9×13 inch baking dish with vegetable oil spray and set aside.

While the oven is preheating, place the diced potatoes, covered in water, in a microwave-safe bowl and microwave them for 4 minutes.

Sauté onions in olive oil over medium heat in a large pan.

When the potatoes are done, drain the water and then add them to the onions. Sauté the onions and potatoes for another 3 minutes and turn off the heat.

In a large bowl, whisk the eggs, milk, and salt together and pour it into the baking dish.

Add the onions and potatoes to the eggs, spreading them out evenly throughout the baking dish.

Add the spinach, mushrooms, and cheese evenly throughout the dish, making sure the ingredients are pushed below the surface of the whisked eggs.

Bake in the oven for approximately 25-27 minutes or until the frittata is very slightly underdone.