Break the three eggs into a bowl and beat them vigorously together with a fork. Season the egg mixture immediately with some freshly ground pepper. Crumble the thirty grams of feta into rough chunks in advance so it's ready for later use.
Heat a splash of olive oil in a frying pan over medium-high heat. Add the fifty grams of spinach to the warm pan. Sauté the leaves, stirring occasionally, for one to two minutes until the spinach has largely shrunk and most of the moisture has evaporated.
Pour the beaten eggs evenly over the spinach in the pan. Let the omelette cook gently over low heat and set. Sprinkle the crumbled feta over the omelette once the bottom is firm and the top looks almost dry.
Fold the omelette gently in half with a spatula. Let it rest in the pan for another half minute so the heat can reach the feta. Then slide the warm egg omelette directly from the pan onto a plate and serve immediately.