Preheat oven to 200C/400F and grease a 12 hole muffin tin with butter.
Mix the sourdough starter, eggs, orange juice, vanilla and oil in a bowl. Whisk together until well combined.
In a separate bowl mix together orange zest, cranberries, green apple, sugar, all purpose flour, salt and baking powder.
Now pour the liquid ingredient mixture into the dry ingredients. Gently fold together until just mixed - don't over mix or the muffins will be tough.
Spoon mixture into greased muffin tin. This recipe will fill a 12 hole muffin (each hole around ¾ full). For larger muffins, use a 6 hole, large muffin tin.
Bake at 200C/400F for approximately 20 minutes or until muffins are golden brown.
Notes
Orange - I have used the zest and juice from one large orange. Depending on how you zest your orange, you may need two. Tins/Muffin Tray - I have used a 6 hole muffin tray for larger muffins - you can use a 12 hole, you'll just get smaller muffins. I prefer to larger muffins as they are closer to a bakery style muffin.