Preheat the oven to 200oC, gas mark 6. Place the sausages and onion on a baking tray and mix in the marmalade and 2 tbsp water, season. Bake for 25-30 minutes until golden.
Meanwhile, toast the baps. Place some watercress on the bottom of the bap and add 2 sausages into each bap, spoon over the onions.