1.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.
2.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.
3.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.
4.Preheat oven to 180°C. Oil a 20cm x 30cm rectangular slice pan.
5.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.
6.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.
You can use sage or thyme instead of rosemary, if you prefer. This bread is best made on the day of serving. Or, make it a day ahead and reheat, wrapped in foil, in a 180°C oven until warm.