Daigaku imo is a classic Japanese dish that literally means ‘university potato’, so-named because its inexpensive but wholesome and calorific ingredients have made it a perennial favourite among cash-strapped students. The flavour is sweet, but not overly so, which means it can be enjoyed both as a dessert or as an afternoon snack.
Wash the sweet potato thoroughly (don’t peel it) and cut it into irregular wedges no larger than 3cm thick. Soak the wedges in cold water for 20–30 minutes to remove the excess starch, then dry completely with kitchen paper or a clean tea towel.
Place the oil, sugar, soy sauce, lime zest and juice in a deep frying pan over a low heat and stir. Add the potatoes to the pan, toss to coat in the sugar mixture, and increase the heat to medium. Place a lid on the pan and leave to heat until you hear it sizzling. Turn the heat down to medium-low and cook for a further 2–3 minutes, then remove the lid and cook for another 10 minutes or so, turning the potatoes frequently to ensure they brown lightly on all sides. The potatoes are done when you can pierce them easily with a chopstick or butter knife.
When the potatoes are tender and nicely browned, turn off the heat and stir through the sesame seeds. Leave to cool slightly, then enjoy them on their own or with vanilla ice cream.