This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
糄 combines 米 (rice) with 扁, which likely contributes the sound. The exact historical development is uncertain.
The left side is rice 米, and the right side is 扁. Picture a flat, thin rice cracker being baked: the rice 米 is flattened 扁 into a thin shape.
For ヘン, imagine a hen pecking at the flat rice cracker: hen -> ヘン, and the hen enjoys the baked rice.