Preheat the oven to 200°C.
Mix flour, butter, egg, and milk in a food processor to form a firm dough.
Roll out the dough and mix in the basil and most of the cheese.
Roll the dough to a 2.5cm thickness.
Cut into rounds using a cutter and place on a baking sheet.
Brush the tops with milk and sprinkle with the remaining cheese.
Bake for 15 minutes until risen and golden.
Fry sliced onions in oil until soft.
Add sugar and caramelize for 3 minutes.
Stir in balsamic vinegar and reduce until syrupy.
Serve the warm scones with the prepared onions.