Heat a tablespoon of the oil in a skillet (or wok) over high heat until smoking.
Add chicken and cook, stirring constantly, until opaque (~ 3-4 minutes). Transfer to a bowl and wipe out the skillet
Heat remaining tablespoon of the oil in the now-empty skillet 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 (~ 3-4 minutes)
Add water chestnuts, scallion whites, and garlic. Cook, stirring constantly until fragrant (~ 1 minute)
Whisk sauce to recombine. Return chicken to skillet and add the sauce, tossing to combine.
Transfer to a serving platter and sprinkle with scallion greens.
Serve with lettuce leaves and hoisin sauce.