Place onion in a large glass or ceramic bowl. Pour over apple cider. Set aside for 1 hour to soak.
Preheat a barbecue hotplate on medium-high heat.
Drain onions, reserving ¼ cup cider. Combine balsamic vinegar, sugar and reserved cider in a jug. Stir until sugar has dissolved.
Cut 4 x 50cm pieces of foil. Top each with a 20cm piece of baking paper. Place 3 onion halves, cut-side down, on the centre of each stack. Top each with thyme and bay leaves. Season with salt and pepper. Fold up edges of foil to enclose onions, scrunching foil at the top, but still leaving the top open. Carefully pour ¼ of the reserved cider into each parcel. Scrunch foil together at top to seal.
Place a wire rack on 1 half of hotplate. Place parcels on rack. Cook for 20 minutes. Carefully transfer parcels to hotplate. Cook for a further 10 minutes or until onion is very tender (but still holding its shape) and caramelised. Place parcels on plates. Carefully open (steam will escape from parcels). Serve sprinkled with extra thyme.
