Red beans, known as azuki in Japanese cuisine, have a mild sweetness and chestnut-like flavor that complements many Asian-style recipes. The beans, which require a long cooking time to become tender, are typically turned into a sweetened paste used to fill sweet rolls and other desserts. Red bean paste is thick when made properly, but the beans may absorb too much moisture during preparation -- causing a runny paste that requires thickening before it's ready to use.
Combine the beans with 3 cups of water in a medium saucepan and bring to a full boil over medium-high heat. Turn off the heat when the water boils and leave the beans to soak for 24 hours.
Drain the beans, add 3 cups of water to the saucepan and simmer over medium heat for an hour. Stir every 15 to 20 minutes and add 1 to 3 tablespoons of water as necessary so the beans don't dry out as they simmer.
Turn off the heat when the beans are tender and easy to mash. Boil for an additional 10 to 15 minutes, if necessary, until all the water is absorbed. Excess water makes the bean paste runny.
Mash the beans to a paste using a potato masher or food processor. Force the paste through a wide-mesh sieve to sort the bean skins from the paste. Discard the skins.
Place the bean paste in the center of a triple-layer square of cheesecloth if there is still too much liquid in the paste. Gather the cheesecloth around the paste and squeeze out excess moisture with your hands.
Add the thickened bean paste and sugar in a bowl. Stir until thoroughly combined.