Preheat the oven 210°C / 410°F / Gas 6 ½.
Lightly grease one large baking tray.
Heat the oil in the pan, add the onions and garlic and fry gently over moderate heat for 3 minutes until softened and transparent. Add the crumbled duileasc towards the end and mix gently to combine. Set aside to cool.
Sift the flour and baking powder into a mixing bowl. Stir in the mustard, cream of tartar and cayenne pepper. Dice and rub in the butter with your fingertips until the mixture resembles fine breadcrumbs. Make a well and pour in the milk and egg, reserving a little. Mix gently to combine. Stir in the cheese, and the cooled onion and duileasc mixture.
Turn the dough onto a floured surface and flatten by hand or with a rolling pin, to a thickness of just over 2.5cm (1 inch).
Using a 3.5cm (1 ½ inch) round diameter cutter stamp out scones and place on the baking trays.
Brush with the reserved milk and egg mixture and sprinkle cheese on top.
Bake in the oven for 12 minutes until golden. Remove and cool slightly on a wire rack. Best served warm with butter.
