First make the sauce: Combine orange juice, soy, vinegar or lime juice, sugar, hoisin, and fish sauce and stir well.
Put the oil in a small sauce pan over medium high heat, then add the garlic, ginger and chili flakes. Cook till softened. Add the sauce and simmer till reduced by about one-third.
For the chicken: Mix the cornstarch and egg white together and add the chicken, tossing it to coat.
In a skillet or wok over high, add enough oil so that the chicken will just be submerged. When it's rippling hot, lay the chicken in and cook till crisp, careful to keep each piece from sticking to others. Remove to a paper towel lined plate.
Pour off all but a couple of tablespoons of oil. Return it to the heat and add the chillis, cooking them till they're black, about a minute. Add the garlic, ginger and half the scallions. Stir-fry till softened, another minute or so. Add the sauce and, when the sauce has come to a simmer, the chicken. Toss till well coated. Remove from the heat add the orange zest, stir, then serve, garnishing with the remaining scallion.
Serve with white rice and broccoli rapini blanched and stir-fried with garlic.