Preheat oven to 350°F. Heat a 10-inch oven-safe skillet over medium-high heat for 2 minutes until hot before adding oil.
Add olive oil or butter to the hot skillet, then add mushrooms and diced onions. Sauté for 5-7 minutes until mushrooms are golden brown and all moisture has evaporated. Stir occasionally.
8 oz mushroom, 1 small onion, 2 tablespoons olive oil
Add fresh spinach to the skillet and stir for 1-2 minutes until wilted and bright green. Season vegetables with salt and pepper, ensuring spinach coats mushrooms without excess liquid.
2 cups spinach, 0.5 teaspoon salt, 0.25 teaspoon black pepper
Whisk together eggs, milk, and grated cheese in a bowl. Pour the mixture evenly over the vegetables in the skillet. Tilt the pan gently so eggs spread to the edges. Do not stir after pouring.
6 large egg, 0.25 cup milk, 0.5 cup parmesan cheese
Transfer the skillet to the oven and bake for 20-25 minutes until the edges puff and the center jiggles slightly when the skillet is shaken. Verify internal temperature reaches 160°F. Let rest 5 minutes before slicing.
