Preheat your oven to 350°F (175°C). This will ensure your egg cups bake evenly.
In a skillet, heat the avocado oil over medium heat. Add the finely chopped yellow onion and red bell pepper. Sauté for about 5-7 minutes, or until the vegetables are soft and fragrant. Stir in the baby spinach and cook until wilted, about 1-2 minutes. Remove from heat and let cool slightly.
In a large mixing bowl, crack the 5 large eggs and add the egg whites. Whisk together until well combined. Season with salt and pepper to taste.
Add the sautéed vegetable mixture, chopped cooked chicken, sun-dried tomatoes, and crumbled feta cheese to the egg mixture. Gently fold everything together until evenly combined.
Spray a muffin tin with cooking spray or lightly grease with avocado oil. Pour the egg mixture into each muffin cup, filling them about ¾ of the way full.
Place the muffin tin in the preheated oven and bake for 20-25 minutes or until the egg cups are set and lightly golden on top. A toothpick inserted in the center should come out clean.
Once baked, remove the muffin tin from the oven and let it cool for a few minutes. Carefully run a spatula around the edges of each cup to release them from the tin. Serve warm or allow to cool completely before storing.
