Follow these step-by-step instructions for fluffy, flavorful, and perfect spinach egg muffin cups every time. The process is straightforward and nearly foolproof.

1. Preheat and Prepare: Begin by preheating your oven to 375°F (190°C). Thoroughly grease a 12-cup standard muffin tin with cooking spray or by lightly brushing each cup with olive oil. Don’t be shy with the grease; this is the most important step to prevent sticking. Silicone muffin liners are also an excellent alternative.
2. Combine the Wet Ingredients: In a large mixing bowl or a large liquid measuring cup with a spout (for easier pouring), crack all 12 eggs. Add the milk or heavy cream. Whisk vigorously for about 1-2 minutes until the yolks and whites are completely combined, and the mixture is light, airy, and slightly frothy. This incorporates air, which helps make the muffins fluffy.
3. Add Seasonings and Cheeses: To the whisked egg mixture, add the onion powder, garlic powder, salt, and pepper. Whisk again to combine. Next, stir in the shredded cheddar cheese and the grated Parmesan cheese, reserving a small amount of cheddar for topping, if desired.
4. Incorporate the Spinach: Add the finely chopped fresh spinach to the egg and cheese mixture. Stir gently with a spatula or spoon until the spinach is evenly distributed throughout the batter.
5. Fill the Muffin Tin: Carefully pour or ladle the egg mixture into the prepared muffin cups. Fill each cup about two-thirds to three-quarters of the way full. Be careful not to overfill, as the eggs will puff up significantly while baking. If you reserved some cheddar cheese, sprinkle it over the top of each muffin cup now.
6. Bake to Perfection: Place the muffin tin on the center rack of your preheated oven. Bake for 18-22 minutes, or until the muffins are puffed up, golden brown around the edges, and the center is set. You can test for doneness by inserting a toothpick into the center of a muffin; it should come out clean.
7. Cool and Remove: Once baked, remove the muffin tin from the oven and let it cool on a wire rack for at least 5-10 minutes. The egg muffins will deflate slightly as they cool; this is completely normal. To remove them, gently run a thin silicone spatula or a butter knife around the edge of each cup to loosen it, then lift it out.