Remove the sausages and brush with maple syrup. Pop the baguettes onto the same baking tray. Increase the oven to 220C/200C fan/gas 7 and return the tray for 5-8 mins until the sausages are dark, shiny and cooked through. Stir the mustard, sugar and vinegar into the onions until the sugar has melted. Cut the baguettes open across the top and put 2 sausages into each. Spoon over the mustardy onions and serve.