Using a small sharp knife, remove the seeds and fleshy pulp from the centre of each cucumber slice. Further hollow the cucumber with a teaspoon so that you have tubes with a roughly 5mm shell. Lightly dust the hollow interiors of the slices with the cornflour. Mix all the stuffing ingredients together in a bowl with 1 tsp salt and pepper, and stuff each cucumber ring with the mixture.
Heat a wok or large frying pan, then add the groundnut oil. When it is moderately hot, add some of the stuffed cucumber rings. Cook slowly until they are slightly browned, then turn them over and brown the other side. You may have to do this in several batches, adding more oil if necessary.
When the cucumber rings are browned, remove them with a slotted spoon and place on a plate. When you have fried all the rings, wipe the wok or pan clean.
Reheat the wok or pan, add the sauce ingredients and bring to a simmer. Add the stuffed cucumber rings, cover the pan with a lid and simmer slowly for 7 mins or until the cucumbers are completely cooked. Lift them out of the sauce with a slotted spoon and transfer to a warm serving platter.
Return the pan to the heat and reduce the sauce by a third over a high heat. Add the sesame oil and coriander, then pour the sauce over the stuffed cucumbers to serve.