Step 1

Mix chicken, 2 tablespoons vegetable oil, cornstarch, 2 teaspoons soy sauce, 2 teaspoons sherry, garlic, and ginger in a bowl. Whisk 3 tablespoons soy sauce, 3 tablespoons sherry, oyster sauce, and sambal oelek in another bowl.

Step 2

Heat 1 tablespoon vegetable oil in a wok or large skillet over medium-high heat; cook and stir chicken in batches until golden and no longer pink in the center, about 5 minutes. Transfer cooked chicken to a platter. Heat remaining 1 tablespoon vegetable oil in the same wok or skillet; cook and stir broccoli and onion until broccoli is partially cooked but still firm, 2 to 3 minutes. Stir in oyster sauce mixture and continue cooking until broccoli is tender, about 5 minutes more.

Step 3

Return chicken to wok and add green onions; toss until heated through and coated with sauce. Season with salt and black pepper.