Pre-heat the oven to 175°C (350°F) and grease a medium-large loaf tin.
Melt the butter and set to one side.
In a large bowl, mix the flour, baking powder, baking soda and sugar together. Add the chopped red onion and crumble in the feta.
In a separate bowl, beat the egg and add the yoghurt and milk.
Add the wet ingredients to the dry and stir only until justs combined (the dough will be sticky).
Pour into the prepared loaf tin and sprinkle with a little mozzarella (or cheese of your choice).
Bake for around 35-40 minutes (start to check at the 30 minute mark). The bread is ready when an inserted skewer comes out clean. Leave to cool and then slice. Enjoy!