Heat a pan to medium high heat and add 1 tbsp oil. Add the sausies and cook until caramelised and golden on all sides. Then brush with maple syrup or honey. Meanwhile, heat 2 tbsp oil in a frying pan and cook the onions and mustard seeds until soft and golden. Add the mustard, sugar and vinegar and stir until caramelised.
Heat the oven to 200°C. Warm the rolls through for 5 minutes. Add a sausage in each roll, spoon over the caramelised mustardy onions and serve.