Cut the chicken thighs into bite-sized pieces and season with salt and pepper.
In a large mixing bowl, whisk together the all-purpose flour, water, egg, cornstarch, baking powder, and a pinch of salt to form a batter.
Heat oil in a frying pan or wok over medium-high heat until it reaches around 350°F (175°C).
Dip each piece of chicken into the batter, allowing excess to drip off, then carefully place in the hot oil. Fry in batches for 4-5 minutes or until golden brown. Use a slotted spoon to remove the chicken and drain on a parchment-lined baking sheet.
In another bowl, combine the orange juice, orange zest, sugar, soy sauce, rice vinegar, minced garlic, ginger, and red pepper flakes (if using). Stir to combine.
In the same pan, discard excess oil, leaving about 1 tablespoon. Add the orange sauce mixture and bring to a simmer over medium heat for about 2-3 minutes until slightly thickened.
Add the fried chicken pieces to the sauce and toss to coat evenly. Cook for an additional 1-2 minutes.
Serve hot, garnished with chopped green onions.