In a medium glass bowl, whisk together the whole eggs, liquid egg whites, sea salt, and black pepper vigorously until the mixture is frothy and well-aerated.
Place a non-stick skillet over medium heat and add the olive oil, swirling to coat the surface.
Add the sliced mushrooms to the pan and sauté for 3-4 minutes until they are tender and golden brown.
Toss in the fresh spinach and finely diced cooked chicken breast, stirring for 1 minute until the spinach has just wilted.
Pour the egg mixture over the vegetables and chicken, tilting the skillet to ensure the eggs reach every edge of the pan.
As the base begins to set, use a silicone spatula to gently lift the edges, allowing the raw egg to run underneath.
Once the center is almost firm, sprinkle the fresh chives over the top and fold the omelette in half.
Cook for 30 more seconds to finish the center, then carefully slide the omelette onto a plate and enjoy immediately.