Place 2 tablespoons soy sauce, 1 tablespoon oyster sauce, 1 tablespoon vegetable oil, 1 ½ teaspoons cornstarch, 1 teaspoon granulated sugar, and ¼ teaspoon ground white pepper in a medium bowl and whisk to combine.

Cut 1 ½ pounds boneless, skinless chicken thighs into 1-inch pieces, add to the bowl, and mix until combined. Let marinate at room temperature for 15 minutes, or cover and refrigerate for up to 24 hours. Meanwhile, make the sauce and prepare the vegetables.

Place ¼ cup water, 1 tablespoon chicken bouillon powder, 1 tablespoon oyster sauce, 2 teaspoons soy sauce, 1 tablespoon dark soy sauce, 1 tablespoon Sriracha, 1 tablespoon rice vinegar, 1 tablespoon toasted sesame oil, 2 teaspoons granulated sugar, and 2 teaspoons cornstarch in a small bowl and stir until the cornstarch is suspended.

Trim and cut 1 large bell pepper into 1-inch pieces (about 1 ½ cups). Cut 4 medium celery stalks crosswise on a slight diagonal into ½-inch pieces (about 1 ½ cups). Thinly slice 4 medium scallions (about ½ cup).

Heat 3 tablespoons vegetable oil in a large frying pan (at least 12 inches) or wok over medium-low heat until shimmering. Add 1 cup raw cashews and 8 to 10 dried red chilies. Cook, frequently tossing, until the cashews and chiles are toasted, 3 to 4 minutes. Using a slotted spoon or spider, transfer the cashews and chilies onto one side of a large plate.

Increase the heat to medium high. Add half of the chicken and arrange into a single layer. Sear undisturbed until the bottom is deeply browned (be careful, as it will splatter), 3 to 4 minutes per side. Give it a quick stir and cook until the chicken is just cooked through, 1 to 2 minutes. Using the slotted spoon, transfer the chicken to the other side of the plate. Add the remaining chicken to the pan and repeat cooking and transferring it to the plate.

Add the bell peppers and celery to the pan and cook, tossing every minute or so and scraping up any browned bits from the bottom of the pan, until the vegetables are just starting to soften, about 4 minutes.

Return the chicken, cashews, and chilies to the pan and toss to combine. Stir the sauce again to suspend the cornstarch, then pour into the pan. Stir-fry until the sauce coats the chicken and vegetables, is glossy, and starts to thicken, about 1 minute. Add the scallions and 1 tablespoon toasted sesame seeds, and toss to combine. Serve with steamed rice.