Start by heating olive oil in a pan over medium heat. I sautéed onions, garlic, mushrooms, and spinach until soft and fragrant. This step removes excess moisture and enhances flavor.
In a large bowl, I whisked the eggs with milk, salt, and pepper until smooth. This creates a light and fluffy texture. Make sure everything is well combined.
Once the vegetables cooled slightly, I mixed them into the egg mixture. Then I added the shredded cheese for that rich, melty goodness.
I lightly greased the muffin tin and poured the mixture into each cup. Fill them about three-quarters full to allow space for rising.
Bake in a preheated oven at 180°C (350°F) for 20–25 minutes. I checked them by inserting a toothpick—it should come out clean when done.
Let the muffins cool for a few minutes before removing them. I found they hold their shape better once slightly cooled and are easier to handle.