Preheat the oven to 180c/350f and line a muffin tray with 12 regular liners or 6 jumbo liners.
Add the bananas to a large bowl and mash until smooth. Stir through the mandarin juice, oil, coconut yogurt, apple cider vinegar and vanilla, if using.
Add the flour, oats, bicarb soda and baking powder, along with the chopped mandarin pieces. Fold gently until just combined.
Divide the batter evenly between the muffin liners.
Press two mandarin halves gently into the top of each muffin, leaving the fruit slightly exposed.
In a small bowl, mix together the crumb topping ingredients until clumps form. Sprinkle generously over each muffin.
Bake for 18-20 minutes for 12 regular muffins or 25-30 minutes for 6 jumbo muffins, or until risen, lightly golden and a toothpick inserted into the centre comes out clean.
Allow to cool in the liners for 10 minutes before transferring to a cooling rack. These feel softer than normal when cooling as they have the extra wetness from the mandarin chunks, and mashed banana to keep them soft for days, but they will continue to firm up as they cool.