Toasted mochi are served in a sweet soup of an, a chunky paste made from azuki beans. In the case of zenzai, the rough tsubu-an is used, with still whole beans left in it. When using the thinner pureed version of the bean paste called koshi-an, the resulting dish is not called zenzai, but shiruko.
Delicious on cold winter days, but also an elegant dessert. Served in Japanese-style tearooms (kanmi-dokoro). Zenzai can also be bought ready-made in supermarkets.
[Photo from Japanese Wikipedia]