Add the remaining 2 tablespoons of oil to the wok and heat until smoking. Add the scallions, garlic and salt and stir-fry until the garlic is lightly golden, about 45 seconds. Add the chicken in 3 batches, waiting a few seconds between additions, and stir-fry until it barely turns white, about 2 minutes. Add the mushrooms, corn, jalapeños and green beans and stir-fry until the vegetables are crisp-tender, 2 to 3 minutes. Add the oyster sauce mixture and stir-fry until the chicken is cooked through, about 2 minutes longer. Stir in the basil, transfer to a platter and serve.