Prep and Coat Chicken: Pat chicken thighs completely dry and cut into 1-inch pieces. In a medium bowl, toss the chicken with the beaten egg until fully coated. In a separate shallow dish, place ½ cup of the cornstarch. Working in batches, transfer the egg-coated chicken to the cornstarch and toss until lightly and evenly dusted.

Fry Chicken: Heat the neutral oil in a large skillet or wok over medium-high heat until shimmering. Carefully add half of the coated chicken in a single layer, ensuring not to overcrowd the pan. Fry for 3-4 minutes per side until golden brown, crispy, and cooked through. Transfer the cooked chicken to a wire rack and repeat with the remaining chicken.

Sauté Vegetables: Carefully pour out all but 1 tablespoon of oil from the skillet. Return the skillet to medium heat and add the chopped onion and bell peppers. Sauté for 4-5 minutes, stirring occasionally, until they are tender-crisp. Add the minced garlic and cook for 1 more minute until fragrant.

Make the Sauce: While the vegetables are cooking, whisk together the pineapple juice, rice vinegar, brown sugar, ketchup, and soy sauce in a small bowl. In a separate tiny bowl, mix the remaining ¼ cup of cornstarch with 2-3 tablespoons of cold water to create a smooth slurry.

Thicken the Sauce: Pour the prepared sauce mixture into the skillet with the sautéed vegetables. Bring the sauce to a gentle simmer, stirring constantly. Once bubbling, slowly whisk in the cornstarch slurry. Continue to simmer and stir for another 1-2 minutes until the sauce becomes thick, glossy, and translucent.

Combine and Serve: Reduce the heat to low. Add the fresh pineapple chunks and the crispy chicken back into the skillet. Gently toss everything together until the chicken and pineapple are completely coated in the sauce. Let it heat through for just a minute, then serve immediately. Garnish generously with sliced green onions and toasted sesame seeds.