Preheat the oven: Heat your oven to 200°C (400°F). Lightly grease a large baking dish or sheet pan.

Coat the chicken: Season the chicken with salt and pepper. Dip each piece into the beaten eggs, then coat thoroughly with cornstarch.

Arrange the chicken: Place the coated chicken pieces in a single layer on the prepared baking sheet. Drizzle lightly with vegetable oil.

Bake: Bake for 15 minutes, then turn the chicken pieces and bake for another 8–10 minutes until golden and cooked through.

Make the sauce: While the chicken cooks, whisk ketchup, rice vinegar, brown sugar, pineapple juice, tamari, cornstarch, garlic, and ginger in a saucepan. Bring to a gentle simmer and cook for 3–4 minutes, stirring until glossy and slightly thickened.

Add vegetables: If using pineapple, bell peppers, and onion, add them to the baking dish during the final 5 minutes of baking.

Combine: Remove the chicken from the oven and pour the warm sweet and sour sauce over it. Toss gently until every piece is coated.

Serve: Serve immediately over steamed white rice or cauliflower rice. Garnish with sliced green onions or sesame seeds if desired.