蒸 is a phono-semantic compound of the semantic 艸, which is grass, at top and 烝 at the bottom. Mnemonic tip: I think of the four lines at bottom as lines of steam.
Steaming is one of the main cooking techniques in Chinese cuisine (alongside stir-frying and deep-frying), especially in dim sum, as evidenced by the stacks of bamboo steaming baskets.