Preheat oven to 180 C (355 F) and line two mini muffin trays with paper muffin liners.
Put banana and watermelon together in a food processor and blend until smooth.
Pour banana and watermelon into a mixing bowl. Add egg and lightly whisk to combine.
Add flour, mix until it forms a smooth batter.
Put spoonfuls of the mixture into each muffin cup so they are 3/4 full.
Bake for approximately 15 minutes.