Mix together the oil, garlic and rosemary. Season. Split the ciabatta in half widthways, without cutting all the way through. Open out flat, then scoop out a little of the bread from both bottom halves.
Brush most of the rosemary oil over the cut sides, then spread the base of each with 2 tbsp of the onion relish and top with slices of cheese. Drizzle with the remaining oil, then fold down the top halves of the loaves. Tightly wrap in foil and chill until needed; it can be made a day ahead.
Preheat the oven to gas 6, 200°C, fan 180°C. Bake for 25 mins, until the cheese melts.