Step 1

Combine yeast and milk in a jug. Sift flour and 1½ tsp salt into a large bowl. Stir in yeast mixture and ¼ cup oil; mix to a firm dough. Transfer dough to a lightly floured surface and knead until smooth and elastic (5 minutes). Place dough in an oiled bowl, cover and stand in a warm place until dough has doubled in size (1 hour).

Step 2

Meanwhile, heat ¼ cup of the remaining oil in a large saucepan over medium heat. Add onion and cook, covered, stirring occasionally, until onion is soft (30 minutes). Stir in garlic, thyme and cook, uncovered, stirring occasionally to combine (5 minutes). Season to taste. Spread onion on a tray to cool.

Step 3

Meanwhile, preheat oven to 180°C. Oil a 20cm x 30cm tray. Turn dough onto a lightly floured surface and knead until smooth. Press or roll dough into a 25cm x 36cm rectangle on a lightly floured sheet of baking paper. Spread onion mixture on dough, top with cheddar and three-quarters of the Gruyère. Turn paper so a long side of the dough is in front of you. Roll up dough from the long side, using paper as a guide; cut crossways into 12cm x 3cm thick slices. Arrange slices, cut-side up, in pan (three across, four down). Stand, uncovered, in a warm place until dough has risen slightly (15 minutes).

Step 4

Top rolls with remaining oil. Bake for 20 minutes. Top with remaining Gruyère and bake until deep-golden and cooked (10 minutes; cover loosely with foil if rolls brown too much). Serve warm.