。
She told her boss what she was thinking.
Literal
She [topic-は] thinking thing [object-を] superior [to-に] told.
思うこと ('the thing one thinks / what one is thinking') is a headless relative — こと names the abstract content of thought without a separate head noun. The pattern works the same way as 言うこと ('what is said'), 望むこと ('what one wants'). 上司 ('one's superior, boss') is the workplace term for someone above you in the hierarchy — distinct from 部長 (specific title) or ボス (loanword, casual). Speaking honestly to one's 上司 is a fraught social act in many Japanese workplaces.