Place the washed beans in a medium pot and add enough water to cover them by about 2 inches.

Bring to a boil over medium-high heat.

Once boiling, reduce the heat to maintain a steady simmer. Partially cover the pot and cook, stirring occasionally, until the beans are nearly tender and most of the water has evaporated. This will take at least 1 hour, possibly longer.

Add more water as needed to keep the beans submerged, but use it sparingly towards the end.

Add the sugar, mirin, and soy sauce to the pot. Increase the heat slightly and continue to cook, stirring frequently, until the liquid reduces to a thick, glossy glaze that coats the beans. The beans should be firm-tender, not mushy.

Remove the pot from the heat and stir in the dark sesame oil.

Just before serving, toast the sesame seeds in a small, dry skillet over medium heat for 1-2 minutes, shaking the pan until they are fragrant and lightly colored.

Sprinkle the toasted seeds over the beans.

Serve warm, at room temperature, or chilled.

Keep a close eye on the beans once you add the soy sauce and sugar. The glaze can go from perfect to burnt very quickly, so stir frequently as it thickens.