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Preheat the oven to 375°F (180°C) and fill a 12-cup muffin pan with 9 paper liners. Set aside.
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In a medium bowl, combine the egg, bananas, milk, apples, carrots, butter and vanilla extract.
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In a separate medium bowl, combine the flour, oats, cinnamon, baking powder, baking soda and salt.
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Mix the wet and dry ingredients together until just combined, then divide equally among the prepared muffin cups (they will be filled to the top).
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Bake for 25-30 minutes or until a cake tester inserted into the center comes out clean.
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Let cool for 10 minutes in the pan, then transfer to a wire rack to cool fully.
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Store at room temperature in an airtight container for up to 2 days. For longer storage, freeze for up to 2 months.