Sear the chicken
Season chicken with salt and pepper. Heat oil in a large skillet over medium-high heat. Add chicken and cook until golden and mostly cooked through. Remove and set aside.
Make the sauce
Reduce heat to medium. Add butter and garlic to the skillet and sauté until fragrant, about 30 seconds. Stir in brown sugar, soy sauce, and pineapple juice.
Add pineapple
Add pineapple chunks and simmer for 2–3 minutes until the sauce begins to thicken.
Finish cooking
Return chicken to the skillet. Toss to coat evenly and simmer for 5–7 minutes until the sauce is thick, glossy, and the chicken is fully cooked.
Serve
Garnish with green onions or sesame seeds and serve warm.