Tetsunabe no Jan! Gogyou-kun no Tanoshii Hong Kong Seikatsu

Gyoukai Go, who lives in the Kowloon district of Hong Kong, is known as "Reverse Wuxing," as he specializes in cooking dishes based on traditional Chinese medicine, which kills those who eat it. One day when a job comes in for him, he plots to assassinate his target with medicinal cooking!

(Source: Kadokawa, translated)