Prep: Place the chicken thighs in a single layer at the bottom of the slow cooker. Top with the pineapple chunks and sliced onions.
Sauce: In a small bowl, whisk together brown sugar, soy sauce, garlic, and ginger until the sugar is mostly dissolved. Pour over the chicken.
Cook: Cover and cook on LOW for 4–5 hours (or HIGH for 2–3 hours) until the chicken is tender and fully cooked.
Thicken (Optional): For a glossier, thicker sauce, whisk 1 tbsp cornstarch with 1 tbsp water and stir it into the pot 30 minutes before serving.
Serve: Garnish generously with toasted sesame seeds and sliced green onions. Serve hot over a bed of fluffy white rice.