Combine yeast, 1 teaspoon of the sugar and the milk in a medium jug. Stand in a warm place for 15 minutes or until mixture is frothy.

Sift flour and salt into a large bowl. Stir in yeast mixture and ¼ cup (60ml) of the oil; mix to a firm dough. Knead dough on a floured surface for 5 minutes or until smooth and elastic. Place dough in an oiled large bowl; cover with plastic wrap. Stand in a warm place for 1 hour 20 minutes or until dough has doubled in size.

Meanwhile, heat ¼ cup (60ml) of the remaining oil in a large saucepan over medium heat, add onion and remaining sugar; cook, covered, stirring occasionally, for 30 minutes or until onion is soft. Stir in garlic, rosemary and vinegar; cook, uncovered, stirring occasionally, for 5 minutes or until liquid is evaporated. Season to taste. Spread onion mixture on a large tray to cool.

Preheat oven to 180°C. Oil a 20cm x 30cm rectangular slice pan.

Turn dough onto a lightly floured surface; knead until smooth. Press or roll dough into a 25cm x 36cm rectangle on a lightly floured sheet of baking paper. Turn paper so a long side of the dough is in front of you. Spread onion mixture on dough; top with the cheddar and three-quarters of the mozzarella. Roll up dough from the long side, using paper as a guide. Cut into 12 x 3cm thick slices. Arrange slices, cut-side up, in pan (three across, four down). Stand, uncovered, in a warm place, for 15 minutes or until dough has risen slightly.

Top pull-apart with extra rosemary and remaining oil. Bake for 20 minutes. Top with remaining mozzarella; bake for a further 10 minutes or until browned and pull-apart sounds hollow when tapped. Cover loosely with foil if it starts to overbrown.