Liberally grease a 4-hole Yorkshire pudding tray, with holes 8 cm (3 1/2 inches) across. Divide the onion and bacon between the holes in the tray and cook in a preheated oven, 240°C (475°F), Gas Mark 9, for 5 minutes.
Meanwhile, place the flour, egg, milk and mustard in a blender, season to taste and blend until smooth.
Arrange the apple in the holes in the tin, then pour over the batter. Return to the oven for a further 20 minutes or until puffed and golden.