Heat 1 avocado oil in a skillet and saute the onion for 4-5 minutes until translucent.
Next add in the chopped spinach and a pink of sea salt. Saute for 7-8 minutes until the spinach has wilted down and released its liquid. Stir in the fresh herbs and cook for 2 minutes longer.
Place the wilted spinach in a colander and press to drain all of the water from the spinach.
Put the spinach, onion and herb mixture in a bowl and gently stir in the feta cheese. Season to taste with salt and pepper.
Make the omelet
Preheat a cast iron griddle or skillet over medium, medium-low heat. Make sure it has had a few minutes to fully heat up.
Crack eggs into a small bowl and whisk to scramble the eggs. Place 1 tbsp of butter on the griddle and smear it around. Wait for the butter to start to foam up and sizzle before pouring on half of the egg mixture.
Cook the omelet for 2-3 minutes or until it has just started to set, then flip it.Now scoop half of the spinach filling into a long strip in the middle of the omelet.
Cook for 1-2 more minutes, then fold the sides over the filling and remove to a plate. Serve hot!
Leftover spinach filling can be stored in the fridge for up to 4 days.