Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 12-cup muffin tin with non-stick cooking spray or line with paper/silicone liners.
Step 2: In a medium skillet, heat the olive oil over medium heat. Add the fresh spinach and cook, stirring occasionally, until completely wilted, about 2-3 minutes. Remove from heat, let cool slightly, then squeeze out as much excess water as possible from the spinach. This prevents watery muffins.
Step 3: In a large bowl, crack the 12 large eggs. Add the milk, salt, and black pepper. Whisk vigorously for 1-2 minutes until the eggs are light, frothy, and well combined.
Step 4: Add the squeezed, wilted spinach and the crumbled feta cheese to the whisked egg mixture. Stir gently to combine, ensuring the spinach and feta are evenly distributed throughout the mixture.
Step 5: Carefully ladle the egg mixture into the prepared muffin cups, filling each cup approximately three-quarters full. Do not overfill, as the muffins will puff up during baking.
Step 6: Bake for 18-22 minutes, or until the egg muffins are set, lightly golden around the edges, and a knife inserted into the center comes out clean. Avoid overbaking to keep them tender.
Step 7: Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. This helps them firm up and retain their shape.
Step 8: Serve warm, or store cooled muffins in an airtight container in the refrigerator for up to 4-5 days. Reheat briefly in the microwave or oven when ready to enjoy.