Preheat oven to 200°C/400°F. Grease a 12-hole (⅓ cup/80ml) muffin pan.
Heat oil in a medium frying pan; cook onion, stirring for 5 minutes or until onion softens. Add spinach; cook, stirring, for 1 minute or until wilted. Cool.
Sift flour into a large bowl; stir in combined butter, egg and buttermilk. Add cheeses and spinach mixture to flour mixture; mix gently to combine. Do not over-mix; mixture should be lumpy. Spoon mixture into pan holes.
Bake muffins for 20 minutes or until a skewer inserted into the centre comes out clean. Leave muffins in pan for 5 minutes before turning, top-side up, onto a wire rack. Serve warm.