Preheat oven to 150°C fan forced. Peel the onions, then cut in half crossways.
Place the onions in a roasting pan, so that they fit snugly together in a single layer. Pour the water into the pan, it should just cover the base.
Press a sheet of baking paper onto the surface then cover tightly with foil. Cook for 1 hour.
Remove the foil and baking paper, drain the liquid. Increase the oven 220C° fan forced. Combine the oil and maple syrup in a jug, pour over the onions then return to the oven. Roast uncovered, for 20 minutes, carefully turning over after 10 minutes or until onions are lightly caramelized.
Meanwhile, place bacon into a non-stick frying pan over medium heat, cook stirring until light golden. Add the breadcrumbs and, cook, shaking the pan until breadcrumbs are light golden. Remove from the heat, stir in the parmesan and season.
Spoon the crumble and thyme over the onions to serve. Serve warm or at room temperature.