Heat 2 tablespoons oil in a large frying pan over medium heat. Cook onion, stirring occasionally, for 10 minutes or until softened. Add sugar. Cook, stirring occasionally, for 5 minutes or until onion is caramelised. Stir in vinegar and thyme. Season with salt and pepper. Cook for 2 minutes. Remove from heat. Cool for 15 minutes.
Preheat oven to 200C/180C fan-forced. Line a baking tray with baking paper.
Place bread on a board. Cut loaf diagonally into 3cm-thick slices, being careful not to cut the whole way through. Repeat cutting in the opposite direction to create diamonds
Combine butter and garlic in a bowl. Carefully spread butter mixture over cut sides of bread. Combine caramelised onion and mozzarella in a bowl. Push ½ the fetta, onion mixture and brie into cuts of bread. Repeat with remaining fetta, onion mixture and brie. Drizzle bread with remaining oil. Season with pepper.
Place bread on prepared tray. Bake for 20 minutes or until bread is golden and cheese has melted. Stand for 5 minutes. Serve sprinkled with extra thyme.
Recipe courtesy of taste.com.au
Serving size 4