Preheat the oven to 175 degrees celsius. Line a baking tray with baking paper.
Slice the croissants in half and dip in the syrup, letting the excess drip off the croissants and place on the prepared tray.
Open the croissants and place two slices of brie, 2 slices of prosciutto and a tbsp of the caramelised onion. Close the croissant and place a small piece of brie and prosciutto on top.
Place in the oven and bake for 15 minutes until the brie has started to melt and the ham is getting crispy.
Remove from the oven and leave to cool slightly before eating!