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Heat the butter in a sauté pan, add the onions, then sprinkle over the sugar. Sweat the onions for 20 mins over a medium heat, stirring them occasionally, until sticky and golden. Add the brandy, if using, reduce down to nothing, then season to taste. This onion topping can be made 2 days ahead and kept covered.
Heat oven to 200C/fan 180C/gas 6. Toast the bread, spread the onions over, then top with slices of cheese. The toasts can be made up 1 hr ahead. Place on a baking tray, then bake for 15 mins until bubbling and golden. Serve each piece of toast on a plate with a sprig of dressed watercress next to it.