Halve the cherry tomatoes and roughly chop the baby spinach to ensure even distribution.
In a medium-sized bowl, whisk together the whole eggs, liquid egg whites, sea salt, and black pepper until the mixture is well combined and slightly frothy.
Heat the extra virgin olive oil in a non-stick skillet over medium heat.
Add the cherry tomatoes to the skillet and sauté for about 2 minutes until they begin to soften and release their juices.
Add the chopped spinach to the skillet and cook for 1 minute until just wilted, then remove the vegetables from the pan and set them aside on a small plate.
Wipe the skillet if necessary and return it to the heat, reducing the temperature to medium-low.
Pour the egg mixture into the skillet, letting it spread evenly across the bottom.
Use a silicone spatula to gently lift the edges of the omelette as it sets, tilting the pan to allow the uncooked egg to flow underneath.
Once the eggs are mostly set but still slightly moist on top, place the sautéed vegetables and crumbled feta cheese on one half of the omelette.
Fold the other half of the omelette over the filling and cook for 1 more minute to allow the cheese to soften.
Slide the omelette carefully onto a plate and serve immediately while hot.