Follow these detailed steps carefully to ensure your egg white muffins are perfectly cooked, flavorful, and easy to remove from the pan. The key is in preparing the fillings properly before adding the egg mixture.

1. Preheat and Prepare Your Pan: Begin by preheating your oven to 375°F (190°C). This is a crucial first step, as a hot oven helps the eggs cook quickly and evenly, giving them a nice puff. Thoroughly grease a standard 12-cup muffin tin with cooking spray or by brushing it with olive oil. Be generous and ensure you cover the bottom and all sides of each cup. Eggs are notorious for sticking, and this step will save you a lot of frustration later. Silicone muffin liners are also an excellent non-stick alternative.
2. Sauté the Aromatics: Heat the 1 tablespoon of olive oil in a medium-sized skillet or frying pan over medium heat. Once the oil is shimmering, add the finely diced red bell pepper and yellow onion. Sauté for 5-7 minutes, stirring occasionally, until the vegetables have softened and the onion is translucent. This cooking process releases their natural sugars and builds the first layer of flavor.
3. Wilt the Greens: Add the minced garlic and chopped fresh spinach to the skillet. Continue to cook and stir for another 2-3 minutes. The garlic will become fragrant (be careful not to burn it), and the spinach will wilt down considerably. Cooking the spinach is a vital step; it removes excess water, which can otherwise make your finished egg muffins soggy and watery. Once wilted, remove the skillet from the heat and set it aside to cool slightly.
4. Create the Egg Base: In a large mixing bowl or a 4-cup liquid measuring cup (which makes for easier pouring), combine the liquid egg whites, unsweetened almond milk, chopped fresh dill, black pepper, and salt. Whisk everything together vigorously for about 30-60 seconds. You want to incorporate some air into the mixture, which will help the muffins become light and fluffy as they bake.
5. Combine the Ingredients: Once the cooked vegetable mixture has cooled for a few minutes (so it doesn’t scramble the eggs), add it to the egg white mixture in the bowl. Add the crumbled feta cheese as well. Gently stir everything together until all the ingredients 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 muffins will puff up significantly in the oven as they bake, and leaving some space at the top prevents them from spilling over and making a mess.
7. Bake to Perfection: Place the muffin tin in the center rack of your preheated oven. Bake for 20-25 minutes. The muffins are done when they are puffed up, golden brown around the edges, and the center is firm to the touch. To be absolutely sure, you can insert a toothpick into the center of one of the muffins; if it comes out clean, they are ready.
8. Cool and Remove: 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. This cooling period allows them to set further and makes them much easier to remove from the pan. To remove them, carefully run a thin knife or a small silicone spatula around the edge of each muffin cup to loosen it before lifting it out.