Pat chicken dry, season with salt and pepper. Heat oil in a large skillet over medium-high heat and sear chicken in a single layer for 3–4 minutes per side until golden. Transfer to a plate.
In the same pan, add pineapple chunks and cook for 2–3 minutes until browned on the edges.
Reduce heat to medium. Add soy sauce, pineapple juice, brown sugar, rice vinegar, ginger, and garlic. Bring to a simmer, then whisk in the cornstarch slurry. Cook 1–2 minutes until thick and glossy.
Return chicken to the pan and toss to coat in the sauce. Let everything bubble together for another minute. Finish with toasted sesame oil if using, and sprinkle with green onions and sesame seeds.
Spoon over steamed rice with a side of vegetables, or use as a filling for wraps or bowls.
Notes
Use thighs for juicier results; breasts work fine but watch cooking time to avoid drying out.
Fresh pineapple caramelizes better; canned pineapple is a convenient substitute.
The cornstarch slurry thickens the sauce quickly—add slowly and stir to avoid lumps.
To make a sheet-pan version, roast chicken and pineapple at 425°F for 15–20 minutes and toss with warmed sauce.