Carefully separate each sheet of spring roll pastry.
Once separated, cover the pile with cling film so as not to let it dry out.
For the marinade, mix together all the marinade ingredients in a shallow dish.
Mix all the remaining ingredients (except the banana and oil) with the marinade. Leave to marinate for at least five minutes, ideally longer.
Cut the banana in half to use as an edible glue stick.
Place some of the filling in the centre of each pastry. Fold the pastry over diagonally and ‘stick’ using the banana. Holding your fingertips over the roll, use your thumbs to roll the pastry slightly to enclose the filling. Fold in the sides and ‘stick’ again. Roll until fully closed and ‘stick’ again.
Heat the vegetable oil in a pan or deep-fat fryer to 170C/340F (Caution: hot oil can be dangerous. Do not leave unattended.)
Carefully place each spring roll into the hot oil – you will need to do this in batches so as not to overcrowd the pan. Once browned and cooked through, carefully remove using a slotted spoon and transfer to a plate lined with kitchen roll. Serve hot.
For the lemongrass sweet chilli sauce, mix all of the ingredients well to combine. Serve the chilli sauce with the spring rolls in a small dipping bowl.