Mix the cornstarch with flour in a large bowl. Mix the cornstarch, flour, and 1 tablespoon of oil. Add the egg, 1 cup oil, and water. Whisk until smooth. The consistency should look like pancake batter.
Mix the chicken cubes into the batter. Cover with a generous amount of batter. Allow it to marinate at room temperature for 10 minutes or up to an hour in the refrigerator (see note).
Heat oil until it reaches 350°F in a saucepan or deep fryer. A wire rack, slotted spoon, or mesh strainer should be set aside.
Fry the chicken in batches in the hot oil. Transfer the chicken to a rack using a slotted spoon. Repeat for remaining chicken.
Heat a wok or skillet on medium heat. Stir in the 1 tablespoon oil, bell peppers, and onion. Cook for 2 to 3 minutes or until they are slightly softened.
Next, add the pineapple to the saucepan. The sauce ingredients are garlic, sugar vinegar, soy sauce, ketchup, and sugar. Bring it to a simmer, and then taste the sauce. You can adjust the sugar and vinegar as needed to achieve the right balance.
In a small bowl, combine the water and cornstarch. Slowly stir the mixture into the saucepan. Reduce heat to medium and simmer for 2 to 3 minutes, or until sauce is thick enough that it coats the vegetables.
Stir the chicken into the pan. The sauce will thicken and become sticky as you take the skillet off the heat.
Serve immediately with optional sesame seed garnish.
