Vegetable combined with celery and carrot to form "mirepoix"

ONION

5-letter answer

This clue refers to a classic cooking base called mirepoix, which is a mix of diced vegetables. The three main ingredients are celery, carrots, and the missing piece—onion. Since the clue names two of them, the answer is the third, making "onion" the straightforward solution. No wordplay here, just a direct culinary reference.