In a large mixing bowl, combine the eggs, milk, salt, pepper, dill, basil, and chicken. Whisk until completely combined.
Heat a 12 inch cast iron skillet over medium heat and add the butter. Once melted, add the red onion and saute for about 3-4 minutes or until translucent. Add the spinach and cook until wilted.
Pour the egg mixture into the skillet and evenly sprinkle the gruyere and feta over the top. Let cook over medium-low heat for about 5 minutes.
Transfer the skillet to the oven and bake for 18-20 minutes or until the eggs are set.
Let cool for about 10 minutes before slicing and serving. Garnish with green onion if desired!
Notes
Make sure to use a 12 inch cast iron or other oven safe skillet. If you use a smaller or bigger size, you'll need to adjust ingredient amounts and bake time accordingly.