Preheat your oven to 375°F (190°C) and grease a standard 12-cup muffin pan with cooking spray or butter.

In a skillet over medium-high heat, add sliced mushrooms and sauté for about 2-3 minutes until golden brown and tender.

Add the roughly chopped spinach to the skillet and cook for an additional minute or until the spinach is wilted.

In a large mixing bowl, crack the room temperature eggs and whisk them until fully combined. Stir in parmesan cheese, milk, garlic powder, onion powder, salt, and black pepper.

Once the sautéed mushrooms and spinach have cooled slightly, gently fold this mixture into the egg base.

Fill each muffin cup about three-quarters full with the prepared mixture.

Place the muffin pan in the preheated oven and bake for 18-20 minutes until they appear set and slightly jiggly in the center.

Allow the muffins to cool in the pan for about 5 minutes before transferring them to a wire rack.