In a medium bowl, whisk the egg white with 1 tablespoon cornstarch and 1 tablespoon soy sauce until smooth.
Add the thinly sliced chicken to the bowl and toss to coat; refrigerate and marinate for 30 minutes.
Heat 1 tablespoon vegetable oil in the wok or skillet over medium heat, then add the marinated chicken and cook 2–3 minutes until opaque; remove and set aside.
Add the remaining 1 tablespoon oil to the pan over medium-high heat, then stir-fry the minced garlic and sliced onion until the onion is translucent, about 2 minutes.
Add the bell pepper and mixed vegetables and stir-fry 3–4 minutes until vegetables are tender-crisp.
Return the cooked chicken to the pan and pour in 1/4 cup soy sauce, 2 tablespoons oyster sauce, and 1 teaspoon sugar; season with salt and pepper to taste and stir to combine.
Pour in 1 cup chicken broth and bring to a simmer, then slowly stir in the cornstarch slurry (2 tablespoons cornstarch mixed with 4 tablespoons water) until the sauce thickens, about 2 minutes.
Remove from heat and serve immediately with cooked rice.