Preheat your oven to 180C (350F). Grease a 20cm (8in) oven proof frying pan or spring form pan. Defrost spinach and squeeze as dry as possible (I push it in a sieve with a spoon).
Add eggs, spinach and crumbled or chopped feta to the prepared pan. Mix to combine. No need to season as the feta adds plenty of salt.
Bake for 15-20 minutes or until frittata is puffy and just cooked through.
Divide between two plates and drizzle with chilli oil or tabasco (if using).