Mix olives, garlic, balsamic vinegar and chilli flakes together in a bowl.
Carefully peel mandarins and cut peels into strips. Squeeze juice from 1 mandarin over olive mixture. Add peel strips to olive mixture and toss well.
Make a packet with a large piece of foil. Pour olive mixture into packet. Arrange onion slices and rosemary over olives; drizzle with olive oil. Sprinkle fennel seeds over mixture. Fold and seal packet; place on a baking tray.
Bake in the preheated oven until warmed, about 90 minutes. Serve warm.