In a heavy based pan add butter and cook the sliced onions gently at a low heat for 40 mins. Stir regularly until soft and caramelised. Once cooked set to the side to cool.
Preheat oven to 200°C.
In a mixing bowl whisk eggs, oil, milk, egg and mustard until well combined. When combined stir through the caramelised onions and the chives.
In a separate mixing bowl combine dry ingredients: flour, cheddar and 25g parmesan.
Add the dry ingredients to wet ingredients and gently combine by folding together. Don’t over mix. It is ok to see a few lumps of flour as this will help the muffins become soft and crumbly when they bake.
Gently spoon the mix into paper muffin cups in a muffin tray and sprinkle remaining 25g grated parmesan cheese on top.
Bake in the oven for 25 -30 minutes, or until golden brown and cooked in the centre. Once baked remove and cool on a wire rack.