If time allows, soak the beans for several hours or overnight, then drain. (If not, proceed, but expect cooking time to be somewhat longer.) Place in a pot, add water to cover, and bring to a boil over medium-high heat. Partially cover, adjust the heat so the mixture simmers steadily, and cook, stirring occasionally, until the beans are nearly tender and most of the water has evaporated, at least an hour and probably more. (Add water as necessary to keep the beans covered, but bear in mind that eventually you will want no water at all, so don’t drown them.)
Add the sugar, mirin, and soy sauce and raise the heat a bit. Continue to cook, stirring frequently now, until the beans are glazed and still firm, not quite as tender as you’re used to; leave them quite moist (the soy sauce will burn if the mixture dries out). Stir in the sesame oil.
You can serve the beans immediately, at room temperature, or chilled. Just before serving, toast the sesame seeds in a small dry skillet over medium heat, shaking the pan frequently until the seeds color slightly. Sprinkle the beans with the seeds and serve.
