Heat the oven to 200C. Line a baking tray with baking paper.
Heat a small heavy-based saucepan over low heat. Add 15g of the measured butter and the oil. As the butter begins to sizzle, add the onions. Cook over low heat, stirring occasionally until lightly caramelised, about 15 minutes. Transfer to a bowl and set aside to cool.
Sift the flour, salt and baking powder into a bowl. Cut the remaining 50g butter into even-sized dice and add to the flour. Using your fingertips rub the butter into the flour, shaking the bowl a couple of times to bring the larger pieces of butter to the surface. Rub until the mixture resembles breadcrumbs. Stir in the cheese.
Pour the buttermilk over the cooled onions and stir to combine. Add to the flour mixture and stir until combined. The mixture will be a little sticky but that's good. Turn dough out onto a lightly floured bench top and press out until 2.5-3cm thickness. Use a 7cm round cutter to stamp out circles. Place on the prepared tray as you go, placing apart as some of the cheese will ooze out during baking and become deliciously crispy. Quickly gather up the scrapes and and cut out again. Sprinkle each with a little flaky salt. Place in the oven and bake for 20-25 minutes until golden brown.
Eat warm on their own or with butter.