Heat remaining 1 tablespoon oil in the now empty pan over high heat until smoking. Add celery and mushrooms; cook, stirring constantly, until mushrooms have reduced in size by half and celery is crisp-tender, 3 to 4 minutes. Add water chestnuts, spring onion whites and garlic; cook, stirring constantly, until fragrant, about 1 minute. Whisk sauce to recombine. Return chicken to skillet; add sauce and toss to combine. Transfer to a serving bowl.