Step 1

Preheat the oven to 400°F.

Step 2

In a medium bowl, whisk together eggs and milk until the egg whites and yolks are completely blended with the milk. Then whisk in kosher salt, fresh ground pepper, and Parmesan cheese.  Set the mixture aside.

Step 3

In a 10-inch oven-safe skillet, heat the olive oil over medium heat.

Step 4

Saute the onions and mushrooms together until the onions are soft and the mushrooms are browned (about 8 minutes).

Step 5

Add the garlic to the pan and sauté for another minute.

Step 6

Add the spinach and sauté until wilted, 2 to 3 minutes.

Step 7

Pour the egg mixture into the skillet.

Step 8

Rearrange the spinach and mushroom mixture in an even layer.

Step 9

Cook for about 5 to 7 minutes over medium heat until the bottom is set and lightly browned. You can check by pulling it up on the side with a spatula. Sprinkle the top with a little more Parmesan cheese.

Step 10

Place the skillet in the oven and bake for 10 to 15 minutes until the top is puffed and slightly browned. The center of the frittata should jiggle slightly when you shake the pan.

Step 11

Let rest for ten minutes, then cut into wedges and serve warm.