。
She was right in the middle of slicing cucumbers.
Literal
She [topic-は] cucumber [object-を] is-cutting [in-middle-of-最中] was.
~ている最中 ('in the middle of doing X') is a tighter, more emphatic alternative to plain ~ている — it locks the action in as in-progress at a specific moment, often interrupted by something else. 最中 ('the height of, peak of') as a noun is a common temporal-marker — 試合の最中 ('in the middle of the game'), 食事の最中 ('in the middle of a meal'). Pairing it with the past だった instead of present だ frames a moment in the past where she was caught mid-cucumber. Useful pattern when describing the moment something interrupts another ongoing action.