Preheat oven to 180°C (or 160°C fan bake). Set aside a 6-mould muffin tin. Cut 6 squares of baking paper to line each muffin hole. Place one square in each hole or alternatively grease muffin tin with olive oil. Bring a kettle to the boil.
Place spinach in a bowl and cover with boiling water, let sit for 1 minute to wilt. Drain spinach and use tongs to squeeze out extra liquid.
Crack eggs into a bowl, add milk, salt measure and whisk, until well combined. Season with pepper.
Divide spinach evenly between each muffin holes. Crumble feta over the spinach. Divide egg mix evenly between each muffin hole and top with sesame za'atar.
Place on middle oven rack and cook for 15-20 minutes, until egg has cooked through.
Enjoy straight away or allow to cool to room temperature, before storing in an airtight container in the fridge for up to 4 days.