Step 1: Coat the Chicken
Pat the chicken pieces very dry with paper towels. In a medium bowl, toss the chicken with the soy sauce, salt, and pepper. If using the egg white, whisk it lightly and mix it in now. Sprinkle the 3 tablespoons of cornstarch over the chicken and toss vigorously until every piece is evenly and thoroughly coated. The coating should look pale and dry.

Step 2: Sear the Chicken
Heat the 2 tablespoons of vegetable oil in a large non-stick or well-seasoned skillet over medium-high heat. Once the oil is shimmering, add the chicken in a single layer, working in batches if needed to avoid crowding. Let it cook undisturbed for 2-3 minutes until golden brown on one side, then flip and cook for another 2-3 minutes until golden and mostly cooked through. The chicken will finish cooking in the sauce. Remove the chicken to a clean plate.

Step 3: Sauté the Vegetables
In the same skillet, add the remaining 1 tablespoon of oil. Add the bell peppers and onion. Stir-fry for 4-5 minutes until they are crisp-tender and starting to caramelize at the edges.

Step 4: Prepare the Orange Sauce
While the vegetables cook, make the sauce. In a medium bowl or measuring cup, whisk together the orange zest, orange juice, brown sugar, soy sauce, rice vinegar, minced garlic, grated ginger, and red pepper flakes (if using). In a separate small bowl, make the slurry by mixing 1 tablespoon of cornstarch with 2 tablespoons of cold water until smooth.

Step 5: Build and Thicken the Sauce
Push the vegetables to the sides of the skillet. Pour the orange sauce mixture into the center. Bring to a simmer. Give the cornstarch slurry a quick re-stir and slowly pour it into the simmering sauce while stirring constantly. The sauce will begin to thicken and turn glossy within 30-60 seconds.

Step 6: Combine and Finish
Once the sauce has thickened to a glaze-like consistency, return the seared chicken (and any accumulated juices) to the skillet. Toss everything together—chicken, vegetables, and sauce—until the chicken is heated through and everything is beautifully coated. Let it simmer together for 1-2 minutes.

Step 7: Garnish and Serve
Remove the skillet from the heat. Garnish generously with sliced green onions and toasted sesame seeds.

Step 8: Serve Immediately
Serve the Sweet Orange Chicken hot directly from the skillet, over a bed of steamed jasmine or white rice. For a complete meal, consider adding a side of steamed broccoli or edamame.