Begin by placing the cubed boneless, skinless chicken breasts in a large mixing bowl and sprinkle with cornstarch. Toss the chicken until every piece is well-coated.

Heat 1 tablespoon of vegetable oil in a large skillet over medium-high heat. Once the oil is shimmering, carefully add the chicken pieces. Fry for 5 to 7 minutes until golden brown.

In the same skillet, add another tablespoon of vegetable oil and toss in the sliced red and green bell peppers and onion. Stir-fry for 3 to 4 minutes until tender-crisp.

Combine vinegar, ketchup, soy sauce, sugar, minced garlic, and grated ginger in a mixing bowl. Stir until the sugar dissolves.

Return the crispy chicken to the skillet with sautéed vegetables. Pour the sauce mixture over and cook for another 2 minutes.

Gently stir in the pineapple chunks, allowing them to mingle. Simmer for about 5 minutes.

To thicken the sauce, mix 1 tablespoon of cornstarch with 2 tablespoons of water and stir into the skillet. Cook for an additional minute until thickened.

Taste and adjust seasoning with salt and pepper as needed.

Serve the dish over jasmine rice or chow mein noodles, garnished with sliced green onions.