Prep the chicken: Pat the chicken dry with paper towels. This helps it sear instead of steam.
Season lightly with salt and pepper.
Make the sauce base: In a small bowl, whisk together soy sauce, pineapple juice, brown sugar, rice vinegar, honey, garlic, ginger, and red pepper flakes if using. Set aside. Stir the cornstarch and water in a separate cup to make a slurry.
Sear the chicken: Heat the neutral oil in a large skillet over medium-high heat.
Add the chicken in a single layer and cook, undisturbed, for 2-3 minutes to get some color. Stir and cook another 3-4 minutes until mostly cooked through.
Add veggies and pineapple: Toss in the red bell pepper and pineapple chunks. Cook 2 minutes to soften the pepper and lightly caramelize the pineapple edges.
Build the teriyaki: Pour in the sauce base.
Bring to a gentle simmer, then stir in the cornstarch slurry. The sauce will thicken within 1-2 minutes. If it gets too thick, add a splash of water or extra pineapple juice.
Finish and brighten: Stir in the sesame oil and most of the green onions.
Taste and adjust salt, sweetness, or acidity. More soy for salt, more pineapple juice or a squeeze of lime for brightness, and a touch more honey if you prefer sweeter.
Serve: Spoon over warm rice and top with remaining green onions and sesame seeds. Add steamed broccoli on the side for a complete plate.