Whisk ¼ cup seasoned rice vinegar, 2 Tbsp. oyster sauce, 2 Tbsp. soy sauce, 2 Tbsp. sugar, 2 tsp. cornstarch, 2 tsp. sesame oil, and ⅓ cup cool water in a small bowl until cornstarch is dissolved; set sauce aside.

Toss 1 lb. skinless, boneless chicken breasts, halved crosswise, sliced against the grain into ½"-thick strips (about the size of french fries), with remaining 1 Tbsp. cornstarch and 1 Tbsp. soy sauce in a medium bowl until chicken is coated (your hands work best here). Set aside.

Heat 2 Tbsp. vegetable oil in a wok or large stainless-steel skillet over medium. Add ⅔ cup roasted cashews, preferably unsalted, and cook, stirring often, until golden brown, about 4 minutes. Using a slotted spoon, transfer to a large plate. (Oil left in pan will be a light brown.)

Increase heat to high. Add 2 bunches asparagus, woody ends trimmed, tips reserved, sliced on a diagonal into ¾"-long pieces, and cook, tossing occasionally, until bright green and charred in spots, about 3 minutes. Using slotted spoon, transfer to plate with cashews.

Add remaining 2 Tbsp. vegetable oil to pan, then add reserved chicken and arrange in a single layer. Cook undisturbed until chicken is golden brown underneath, about 2 minutes. Add 1 bunch scallions, finely chopped, and 6 garlic cloves, finely chopped, and cook, stirring often, until meat is cooked through and no longer pink and garlic is fragrant, 1–2 minutes.

Whisk reserved sauce just to reincorporate any cornstarch that has settled at the bottom of the bowl. Add to stir-fry and cook, stirring often, until thickened and ingredients are coated, about 1 minute. Add cashews and asparagus and stir a few times to coat in sauce. Remove stir-fry from heat; taste and season with kosher salt if needed. Serve stir-fry with steamed white rice if desired.