Velvet the chicken. Add the cut chicken breasts to a large bowl. Beat one egg white and add to the bowl with the chicken. Add 1 tablespoon cornstarch and massage the chicken. Refrigerate while you prepare the other ingredients.
Make the sauce. Add all sauce ingredients, except the cornstarch, to a medium bowl. Whisk together well. Add ½ tablespoon cornstarch and ½ tablespoon of water to a small bowl. Mix the cornstarch and water together to form a slurry. Pour the slurry into the sauce, and mix together well. Set the sauce aside.
Heat the griddle or skillet to medium heat. For griddle cooking, the griddle temp should be about 375-400°F.
Coat the chicken in cornstarch. Take the velveted chicken out of the fridge, and add ⅓ cup cornstarch. Toss together so that each piece is coated.
Cook the chicken. Add about 4 tablespoons of high-heat cooking oil to the griddle or skillet. Allow the oil to heat up. Place the coated chicken in the heated oil, making sure that the chicken pieces are in an even layer so they can shallow fry and get crispy. Allow the chicken to cook on the first side untouched for about 3-4 minutes. Flip the chicken pieces to the second side and cook an additional 3-4 minutes or until chicken pieces are crispy and cooked roughly 80% done.
Cook the vegetables and pineapple. When the chicken is about 80% cooked through, add the chopped vegetables and pineapple chunks to the griddle. Cook until the vegetables are a tender-crisp, about 2 minutes, and then toss together with the chicken.
Add the sweet and sour sauce. Slowly pour about 1/2 of the sauce on the chicken and vegetables. The heat from the griddle or skillet will cause the sauce to bubble up and begin to reduce. Gently mix the chicken and sauce together, and allow the sauce to heat and reduce for about 2-3 minutes. Add the remaining sauce and cook an additional 2 minutes.
Serve the sweet and sour chicken. Serve immediately over white rice, fried rice, or noodles. Garnish with sliced green onions (optional).