Feta and Red Onion Quick Bread (regular or spelt flour)

Name: Feta and Red Onion Quick Bread (regular or spelt flour)
Cuisine: European, Mediterranean, Greek
Ingredients: 50g (¼ cup) butter ((melted)); 250g (2 cups) flour (see note 1); 1½ tsp baking powder; ½ tsp baking soda (bicarbonate of soda); 1 tsp sugar; 1 small red onion (very finely chopped); 50g (1¾ oz) feta cheese; 125ml (½ cup) milk (see note 2); 115g (½ cup) Greek yoghurt (see note 3); 1 egg (medium); sprinkling of mozzarella cheese (see note 4)