Season the chicken with the salt and pepper in a large mixing bowl.
1½ lbs (680 g) boneless, skinless chicken thighs, 1 tsp (6 g) kosher salt, ½ tsp (1 g) white pepper
Add ¼ cup (30 g) of the tempura batter mix to the chicken and toss until lightly coated. This dry coating helps the batter adhere to the chicken during frying.
1¼ cups (150 g) tempura batter mix
Whisk the remaining tempura batter mix, egg, and ½ cup (120 ml) cold water in a medium bowl until a thick batter forms. Add additional water, 1 tablespoon (15 ml) at a time, if needed. The batter should be thick enough to coat the chicken without running off immediately.
1 large egg, ½ cup (120 ml) cold water
Add the chicken to the batter and toss until each piece is evenly coated.
Heat the oil to 350°F (175°C) in a wok or deep skillet.
3 cups (720 ml) neutral oil
Fry the chicken in batches for 5 to 6 minutes, turning occasionally, until golden brown and cooked through. Transfer to a wire rack set over a sheet pan and allow the oil to return to 350°F (175°C) before frying the next batch.
Carefully transfer the oil to a heatproof container, leaving a thin coating in the wok. Wipe out any dark crumbs and return the wok to medium heat.
Add the garlic and ginger and cook for 20 to 30 seconds, stirring constantly, until fragrant.
2 cloves garlic, 1 inch (knob) fresh ginger
Add the orange juice, soy sauce, oyster sauce, sugar, rice vinegar, and hoisin sauce. Bring to a simmer, stirring until the sugar dissolves.
½ cup (120 ml) freshly squeezed orange juice, 2 tbsps (30 ml) low sodium soy sauce, 1 tbsp (15 ml) oyster sauce, ⅓ cup (67 g) granulated sugar, 2 tbsps (30 ml) rice vinegar, 2 tbsps (30 ml) hoisin sauce
Stir the cornstarch slurry, then add it to the sauce. Cook for 1 to 2 minutes, stirring constantly, until the sauce thickens enough to coat the back of a spoon.
1 tbsp (8 g) cornstarch
Return the chicken to the wok and toss until every piece is evenly coated in the orange sauce.
Serve immediately, garnished with green onions and sesame seeds if desired.
green onion, sesame seeds