Gently rinse the adzuki beans, put them in a pot with 700 ml water, and bring to a boil on medium heat. When it boils, add 200 ml water, then bring to a boil again, turn off the heat, and drain.
Return the beans to the pot, add 700 ml water, then simmer on medium heat until tender. The water will boil out, so add water in 3 to 4 batches.
Boil until the beans become plump. To test, scoop the beans with a spatula, and if they can be easily crushed between your fingers, they are done. It should take about 40 to 60 minutes.
Drain tender beans into a colander lined with cheesecloth.
In a pot, add the sugar to the drained beans, and simmer on medium heat.
The beans should produce moisture when the sugar is added. Stir from the bottom of the pot to make sure the beans do not scotch.
Scoop up the beans with a spatula, and when they start to stick to the spatula, add salt, quickly stir, turn off the heat, then transfer to a large bowl to cool.
It's best to let the anko sit until the following day so that the sweetness is evenly distributed.
You can freeze the anko. They are handy if you roll them into little balls to freeze. You can store them for about a month.
To make adzuki bean soup: Add water and sugar to taste and boil. If you put the frozen anko in water and boil, it will melt easily. Add some grilled rice cakes, too!
Try them in ohagi (a type of Japanese wagashi with mochi rice covered in anko).
When making adzuki bean soup from scratch, see Recipe ID: 1670887.
Story Behind this Recipe
I made a stock of anko, so on occasion, I use it to make oshiruko (sweet red bean soup), ohagi, or other kinds of treats.
The total simmering time for the adzuki beans to become tender varies greatly depending on how fresh they are, so check the pot from time to time. They should be simmered until they can be easily crushed between your fingers, and are plump and cooked through. While the beans are simmering, don't stir them up too much.