In a medium bowl, whisk the egg white, cornstarch, and 1/4 teaspoon of the salt until smooth.
Add the chicken chunks to the bowl and toss to coat evenly; let rest 15 minutes at room temperature or refrigerate up to overnight.
In a separate bowl, whisk together 1/4 cup reserved pineapple juice, grated ginger, white vinegar, ketchup, brown sugar, remaining 1/4 teaspoon salt, and cayenne until smooth; set aside.
Heat a wok or large sauté pan over high heat until a bead of water sizzles and evaporates, add 1 tablespoon oil and swirl to coat.
Add the red and yellow bell pepper chunks and stir-fry about 2 minutes until slightly softened; transfer peppers to a plate and wipe the pan dry.
Return the pan to high heat, add the remaining 1 tablespoon oil and coat the surface, then add the coated chicken in a single layer.
Let the chicken fry undisturbed about 1 minute to brown the bottom, flip and cook about 1 more minute; the pieces may still be slightly pink inside.
Add the cooked peppers, pineapple chunks, and the reserved sauce mixture to the pan; stir and simmer about 2 minutes until chicken is cooked through and sauce thickens.
Remove from heat and let rest a minute for the sauce to finish thickening, then garnish with sliced scallions if desired and serve.
