In a medium bowl, whisk the egg with the flour, cornstarch, salt, and pepper until smooth.

Add the chicken and toss so every piece is evenly coated.

Pat the chicken dry for extra crispness before coating.

Heat 2 tablespoons vegetable oil in a large skillet or wok over medium-high heat until shimmering.

Add chicken pieces in a single layer without crowding. Cook 4 to 5 minutes per side until golden brown and crisp.

Transfer cooked pieces to a plate lined with paper towels to drain.

Reduce heat to medium and add the orange juice, sugar, rice vinegar, soy sauce, ginger, garlic, sesame oil, and red pepper flakes. Bring to a gentle simmer.

Stir the cornstarch with a little cold water to make a smooth slurry, then whisk it into the simmering sauce slowly. Keep stirring until the sauce turns glossy and thickens, about 1 minute.

Stir in the orange zest.

Return the crispy chicken to the skillet and toss gently to coat each piece with the sticky orange sauce.

Remove from heat immediately to keep the coating crisp.

Serve the orange chicken over steamed white rice and garnish with sliced green onions and sesame seeds.