Prep the chicken: Pat the chicken dry. Toss with 3 tablespoons cornstarch, 1/2 teaspoon salt, and 1/4 teaspoon pepper until evenly coated.

Make the sauce: In a bowl, whisk ketchup, rice vinegar, soy sauce, brown sugar, and pineapple juice (or water). In a small cup, stir 2 teaspoons cornstarch with 2 teaspoons cold water to make a slurry. Set both aside.

Sear the chicken: Heat 1–2 tablespoons oil in a large skillet over medium-high. Add chicken in a single layer. Cook 5–7 minutes, turning once, until lightly golden and cooked through. Transfer to a plate.

Sauté the veggies: Add another splash of oil if needed. Stir-fry bell peppers and onion with a pinch of salt for 3–4 minutes, until crisp-tender. Add garlic and ginger; cook 30 seconds until fragrant.

Build the sauce: Pour in the sauce mixture. Bring to a simmer. Stir the cornstarch slurry, then add it to the pan, stirring until the sauce thickens and turns glossy, about 1–2 minutes.

Combine and finish: Return chicken to the pan with pineapple. Toss to coat and heat through. Drizzle in sesame oil if using. Taste and adjust—add a splash of vinegar for more tang or a pinch of sugar for sweetness.

Serve: Spoon over warm rice. Garnish with sesame seeds and sliced scallions.