。
The miso soup has warmed up.
Literal
Miso-soup [subject-が] warmed.
温まる ('to warm up by itself') is intransitive — no one is warming the soup, it's warming up on its own. The transitive counterpart is 温める ('to warm something'). 味噌汁 is the quintessential everyday Japanese food word, and reading 汁 as しる ('soup/broth') is one of the common kun-readings of that character.