Preheat your oven to 375 degrees Fahrenheit. While it's warming up, take a large 13” x 9” casserole dish and coat it with cooking spray to prevent sticking.
Next, heat the avocado oil in a large skillet over medium-high heat. Add in the chopped onion and sauté, stirring occasionally, until the onion begins to soften, typically around 5 to 8 minutes.
Once the onion is soft, add the minced garlic and baby spinach. Cover the skillet to allow the spinach to wilt, which should take about 2 minutes. Stir the mixture to combine everything, then add the drained sun-dried tomatoes.
Sprinkle the mixture with sea salt and remove it from heat. This combination will add depth and flavor to your casserole.
Transfer the sautéed chicken and vegetable mixture into the prepared casserole dish, spreading it evenly across the bottom to create a solid base.
In a large bowl, whisk together the large eggs and egg whites until they are well-combined. Season the eggs with a bit more salt to suit your taste.
Pour this egg mixture evenly over the chicken and veggies in the casserole dish, ensuring that all ingredients are coated.
Now it’s time for the star ingredient: crumble the feta cheese and stir it gently into the mixture. This will enrich the dish and give it a creamy texture.
Place the casserole in the center rack of your preheated oven and bake for 35 to 45 minutes. You want it to be golden brown on top and fully cooked through.
To check if it's fully cooked, you can insert a meat thermometer into the center; it should read at least 190 degrees Fahrenheit. After baking, let it cool for at least 10 minutes before slicing.
Serve the Chicken and Spinach Breakfast Casserole warm, perhaps with toppings like sour cream, Greek yogurt, or fresh fruit for a complete meal experience.