Preheat the Oven:

o Preheat your oven to 375°F (190°C).

o Ensure that the oven rack is placed in the center position.

Whisk the Eggs:

o In a large mixing bowl, whisk together the eggs, milk or cream, salt, and a generous amount of freshly ground black pepper.

o Incorporate the ½ cup of freshly grated Parmesan cheese and set the mixture aside.

Sauté the Vegetables:

o Heat the olive oil in a large oven-safe skillet (such as a cast-iron skillet) over medium heat. Introduce the sliced red onion and sauté for 3 to 4 minutes, or until it becomes tender.

o Add the minced garlic and sliced mushrooms to the skillet. Cook for another 5-6 minutes, stirring occasionally, until the mushrooms are softened and have released their moisture.

Add the Spinach:

o Add the fresh spinach leaves to the skillet and cook, stirring gently, until the spinach is wilted and well combined with the mushrooms and onions. This should take about 1-2 minutes.

Pour in the Egg Mixture:

o Pour the egg mixture evenly over the sautéed vegetables in the skillet. Use a spatula to gently stir and distribute the vegetables evenly throughout the eggs.

Cook the Frittata on the Stovetop:

o Allow the frittata to cook on the stovetop for about 2-3 minutes, just until the edges start to set. Do not stir during this time.

Bake the Frittata:

o Transfer the skillet to the preheated oven and bake for 10-12 minutes, or until the frittata is puffed up and the center is just set. If you like a golden-brown top, you can broil the frittata for an additional 1-2 minutes, keeping a close eye on it to prevent burning.

Serve:

o Remove the frittata from the oven and let it cool for a few minutes.

o Sprinkle more grated Parmesan cheese on top if you like. Cut into wedges and enjoy while warm or at room temperature.

o Enjoy your easy and healthy spinach mushroom frittata!