Toss chicken with 2 tsp cornstarch and 1 tbsp soy sauce. Set aside.
Heat 1 tbsp oil. Toast cashews until golden, about 4 minutes. Transfer to a plate.
Increase heat to high. Stir-fry asparagus (tips reserved) until charred, about 3 minutes. Transfer to the plate.
Add more oil if needed. Cook chicken in a single layer until browned, about 2 minutes.
Add scallions and garlic. Cook until chicken is cooked through, about 1–2 minutes.
Whisk together vinegar, oyster sauce, remaining soy sauce, sugar, 1 tbsp cornstarch, sesame oil, and 2 tbsp water.
Add sauce to the pan. Stir until thickened, about 1 minute.
Return cashews and asparagus to the pan. Toss to coat.
Season with kosher salt to taste. Serve with rice.