How to Make It
Heat 2 tablespoons of the oil in a large nonstick skillet over high heat.
Toss the chicken with the cornstarch and ¾ teaspoon salt. Cook, in batches, turning the pieces once, until well browned (but not cooked through), 3 to 4 minutes. Transfer the chicken to a plate; reserve the skillet. Reduce heat to medium.
Add the remaining tablespoon of oil to the reserved skillet. Add the scallion whites, garlic, and ginger. Cook, stirring occasionally, until the scallions are slightly soft, 1 to 2 minutes.
Add as many collard greens to the skillet as will fit. Cook, tossing frequently and adding more greens when there is room, until crisp-tender, 5 to 7 minutes.
Add the soy sauce, cashews, chicken, and 2 tablespoons water to the skillet. Cook, tossing frequently, until the chicken is cooked through and everything is coated with the sauce, 2 to 3 minutes.
Serve over the rice, topped with the scallion greens.