Combine tsubu-an and water into a pot and turn the heat on.
Toast mochi in a toaster oven or similar until browned.
When the soup of the tsubu-an becomes hot, turn the heat off. Transfer into a bowl and serve with mochi.
It's even better if served with takuan (pickled dried and pickled daikon radish) or salted ship-konbu (salted kelp). Enjoy with hot tea.
Story Behind this Recipe
I usually use homemade tsubu-an when making manju, but sometimes I can't use it all up. That's when I make zenzai using leftover tsubu-an. Sometimes I serve it without mochi. The flavor and subtle sweetness of the aduki is still very satisfying.
I created this recipe based on the premise of using homemade tsubu-an, but you could of course use the store-bought variety. If the tsubu-an is unsalted, add a pinch of salt to improve its sweetness. It's also good with mochi cooked in hot water.