Follow these step-by-step instructions carefully to ensure your Spinach Egg Muffin Cups come out perfectly fluffy, flavorful, and cooked through every time.

1. Preheat and Prepare: Begin by preheating your oven to 375°F (190°C). Thoroughly grease a 12-cup standard muffin tin with non-stick cooking spray, olive oil, or butter. This is a crucial step to prevent the eggs from sticking. Alternatively, you can use silicone muffin liners for the easiest release.
2. Sauté the Aromatics: Heat the olive oil in a medium-sized skillet or frying pan over medium heat. Once shimmering, add the finely chopped onion and cook for 3-4 minutes, or until it becomes soft and translucent. Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.
3. Wilt the Spinach: Add the fresh spinach to the skillet with the onions and garlic. It may look like a lot, but it will wilt down significantly. Cook, stirring occasionally, for about 2-3 minutes until all the spinach has wilted. Remove the skillet from the heat.
4. Remove Excess Moisture: Transfer the cooked spinach mixture to a fine-mesh sieve or a colander. Using the back of a spoon or a spatula, press down firmly on the spinach to squeeze out as much excess liquid as possible. This step is critical for preventing watery egg muffins. Set the drained spinach mixture aside to cool slightly.
5. Prepare the Egg Mixture: In a large mixing bowl, crack all 12 eggs. Add the milk, salt, and black pepper. Whisk everything together vigorously until the yolks and whites are completely combined and the mixture is light and slightly frothy.
6. Combine Ingredients: Once the spinach mixture has cooled down a bit (so it doesn’t scramble the eggs), add it to the large bowl with the whisked eggs. Add in 3/4 cup of the shredded cheddar cheese, reserving the remaining 1/4 cup for topping. Gently stir everything together until all the ingredients are evenly distributed.
7. Fill the Muffin Tin: Carefully pour or ladle the egg mixture into the prepared muffin cups, filling each one about three-quarters of the way full. Be sure not to overfill, as the eggs will puff up significantly during baking.
8. Top with Cheese: Sprinkle the remaining 1/4 cup of shredded cheddar cheese evenly over the top of the mixture in each muffin cup. This will create a delicious, golden-brown cheesy crust.
9. Bake to Perfection: Place the muffin tin in the preheated oven and bake for 20-25 minutes. The muffins are done when they are puffed up, the centers are set (not jiggly), and a toothpick or knife inserted into the center of a muffin comes out clean. The edges should be lightly golden brown.
10. Cool and Serve: Remove the muffin tin from the oven and let the egg muffins cool in the tin for at least 5-10 minutes. They will deflate slightly as they cool; this is completely normal. To remove, carefully run a thin knife or spatula around the edge of each muffin to loosen it, then lift it out. Serve warm or allow to cool completely for storage.