Cut the butter into the flour with the salt and mustard powder, then either rub between finger tips or pulse in a food processor until the mixture resembles coarse breadcrumbs.
Stir in the yoghurt or kefir with a knife until the mixture comes together into a ball. You may need to use your hands for the final stage, but be careful not to overwork the dough. If you're using ready made mustard rather than powder, add it at this stage.
Whilst the pastry is resting, fry the onion gently in the olive oil until translucent.
For a vegan version, use vegan pastry instead, either homemade or shop-bought puff pastry.
Will keep for a 3-4 days in the fridge. Can also be frozen.