In a bowl, whisk together the eggs, milk (if using), salt, and pepper until well combined.
Heat the olive oil or butter in a non-stick skillet over medium heat.
Add the chopped spinach to the skillet and sauté for about 1-2 minutes until wilted.
Pour the egg mixture over the spinach and let it cook undisturbed for about 2-3 minutes until the edges start to set.
Sprinkle the crumbled feta cheese evenly over one half of the omelette.
Carefully fold the other half of the omelette over the feta and cook for another 1-2 minutes until the eggs are fully set and the cheese is slightly melted.
Slide the omelette onto a plate and serve immediately.