gruel
饘 centers on a simple, soft food: a thin porridge or gruel made from rice or grains.
饘 combines the food radical 飠 with 亶, which likely provides the sound. The character is associated with gruel or thin porridge.
The food radical 飠 on the left suggests something edible, and the right side 亶 looks like a container with a lid and base. Imagine a pot of thin rice gruel being served.
For セン, picture a thousand (千, せん) grains of rice simmering into a simple gruel: 千 -> セン, and the pot holds the thin porridge.