Preheat the oven to 375 degrees Fahrenheit and spray a large 13” x 9” casserole dish with cooking spray.

Heat the avocado oil in a large skillet over medium-high heat. Add the chopped onion and sauté, stirring occasionally, until the onion begins to soften, about 5 to 8 minutes. Add in the garlic and baby spinach and cover the skillet until the spinach has wilted, about 2 minutes. Stir in the sun-dried tomatoes and sprinkle everything with sea salt. Remove from the heat.

Transfer the chicken and veggie mixture to the prepared casserole dish and spread everything into an even layer.

In a large bowl, whisk together the eggs and egg whites until well-combined. You can season the eggs with salt (which is what I do), depending on your personal taste.

Pour the egg mixture evenly over the chicken and veggies. 

Stir in the feta cheese, moving everything around to ensure the ingredients are well-distributed throughout the casserole. If you'd like, you can sprinkle any type of grated cheese on top of the casserole.

Bake on the center rack of the preheated oven for 35 to 45 minutes or until the casserole is golden brown on top and tests clean.

If you’d like, you can insert a meat thermometer into the center of the egg casserole to be sure it is fully cooked. It is considered fully cooked once it reaches an internal temperature of at least 190 degrees Fahrenheit. For me, this takes 35 minutes of baking time.

Allow the casserole to cool for at least 10 minutes before slicing and serving.

If you’d like, you can serve the individual slices with toppings such as sour cream, plain Greek yogurt, guacamole, or serve with fresh fruit and toasted bread or a green salad for a complete meal.