Heat the avocado oil in a medium non-stick skillet over medium-high heat.
Add the ground turkey to the skillet, breaking it up with a spatula, and cook until browned and cooked through.
Toss in the sliced cremini mushrooms and sauté for 3-4 minutes until they release their moisture and turn a deep golden brown.
In a small bowl, whisk together the whole egg and the three egg whites until the mixture is frothy and well combined.
Season the egg mixture with the sea salt and black pepper.
Reduce the heat to medium and pour the egg mixture evenly over the turkey and mushrooms in the skillet.
Once the edges begin to set, place the baby spinach and crumbled feta cheese over one half of the omelette.
Carefully fold the omelette in half using a silicone spatula and cook for another 1-2 minutes until the cheese is slightly softened and the spinach is wilted.
Slide the omelette onto a plate and garnish with fresh chopped chives before serving.