The weather was fair today. Instead of cooking, my mom brought back some authentic Cantonese food. The congee she brought back was done to a turn. While many people assume making congee is simple, no, the authentic Cantonese version requires great skill. You need to carefully melt the rice at the precise temperature and duration, and the seasoning must be added before it's simmered. The end result is a creamy, white liquid without the graininess from rice, just like Campbell’s milky soup. Alas, it’s easier said than done, but I enjoy thinking about how it's cooked.
done to a turn
idiom
British
: cooked, performed, etc., in a perfect way : done to perfection
The chicken was done to a turn.
I can understand rice melted because it becomes like a Campbell's paste.
We usually say 'for' a duration not 'at' a duration, and I'm not sure the latter is correct.