Follow these detailed steps to achieve perfectly cooked, flavorful, and fluffy egg muffins every single time. The key is in the preparation of the vegetables to avoid a watery final product.

1. Preheat and Prepare Your Pan
First things first, preheat your oven to 375°F (190°C). This ensures the oven is at the perfect temperature to cook the muffins evenly and help them puff up nicely. Thoroughly grease a standard 12-cup muffin tin with non-stick cooking spray or by using a pastry brush to coat each cup with melted butter or oil. Don’t skip this step! Eggs have a notorious habit of sticking, and proper greasing is your best defense for easy removal. Silicone muffin liners are also an excellent, reusable alternative.

2. Sauté the Vegetables: The Secret to Avoiding Soggy Muffins
This is arguably the most crucial step for texture. Heat the olive oil in a medium skillet over medium heat. Add the finely diced red onion and cook for 2-3 minutes until it begins to soften and become translucent. Add the minced garlic and cook for another 30-60 seconds until fragrant. Be careful not to burn the garlic, as it will become bitter.

Now, add the fresh spinach to the skillet. It will look like a mountain, but don’t worry—it wilts down significantly. Cook, stirring frequently, for about 3-5 minutes, until all the spinach has wilted completely.

3. Squeeze Out Excess Moisture
Remove the skillet from the heat. Transfer the cooked spinach and onion mixture to a fine-mesh sieve or a colander. Using the back of a spoon or a spatula, press down firmly on the mixture to squeeze out as much liquid as possible. You’ll be surprised how much water comes out. This step is the key to preventing watery, spongy egg muffins and ensuring a dense, fluffy texture. Set the drained vegetable mixture aside to cool slightly.

4. Whisk the Egg Base
In a large mixing bowl, crack all 12 eggs. Add the milk, salt, and black pepper. Whisk vigorously until the yolks and whites are completely combined, the mixture is uniform in color, and small air bubbles begin to form on the surface. This can take a good 1-2 minutes of whisking by hand. Incorporating air is what helps the muffins become light and fluffy. Using an immersion blender for a few seconds can also create an exceptionally smooth base.

5. Combine All Ingredients
Once the egg mixture is well-whisked, gently fold in the cooled and drained spinach-onion mixture. Add the crumbled feta cheese. Stir everything together until the fillings are evenly distributed throughout the egg base.

6. Fill the Muffin Cups
Carefully pour or ladle the egg mixture into your prepared muffin tin. Fill each cup about three-quarters of the way full. The eggs will puff up significantly during baking, so leaving some space at the top prevents them from overflowing and making a mess in your oven.

7. Bake to Golden Perfection
Place the muffin tin on the center rack of your preheated oven. Bake for 20-25 minutes. The muffins are done when they are puffed up, firm to the touch in the center, and the edges are lightly golden brown. A surefire way to check for doneness is to insert a toothpick or a thin knife into the center of a muffin; if it comes out clean, they are ready.

8. Cool and Serve
Remove the muffin tin from the oven and let it cool on a wire rack for at least 5-10 minutes. The muffins will deflate slightly as they cool—this is completely normal. Running a thin silicone spatula or a butter knife around the edge of each muffin can help loosen them before you lift them out. Serve them warm or allow them to cool completely for storage.