food in tripod, rice porridge
餗 centers on food prepared in a vessel, specifically a tripod, and by extension rice porridge as a typical such dish.
餗 combines 飠 (food) with 束, which likely contributes the sound. The character originally referred to food cooked in a tripod vessel.
The food radical 飠 on the left and 束 (bundle) on the right suggest a bundle of food placed into a tripod to cook, resulting in rice porridge.
For ソク, imagine soaking a bundle of rice in the tripod: soak -> ソク, and the soaked rice becomes porridge.