Lightly whisk the eggs in a small bowl.
Heat the oil in a frying pan, add the spinach and tomatoes, then put a lid on and sauté for a few minutes, until the spinach has wilted. Pour over the egg mixture, spreading it out evenly.
Cook for 5 minutes then lay the anchovies on top and scatter some chilli flakes or ground pepper all over the omelette. Crumble the feta on top, if used.
Put the lid on and cook for another 5-8 minutes, over medium heat, until the egg has set and the cheese has melted.
Scatter some basil leaves on top, fold in half and slide it to a plate. Cut in half and serve.