。
I took up tea ceremony when I retired.
Literal
Retirement [object-を] opportunity [at-に] tea-ceremony [object-を] started.
The pattern ~を契機に is a set formal expression meaning 'taking X as an opportunity / with X as a catalyst.' 契機 is 'opportunity, trigger, turning point'; the full construction is used for pivotal life events leading to new habits or directions. Functionally close to ~をきっかけに. Subject omitted — context supplies the speaker.