In a large bowl, mix the sugar, ketchup, pineapple juice, soy sauce, paprika, Worcestershire sauce, apple cider vinegar, and ground black pepper until evenly combined.
Transfer 1/2 cup of this marinade to a small container and store in the fridge (you will use this to brush the chicken later on). Transfer the rest of the marinade to a large Ziploc bag and mix in the minced garlic and ginger. Add the chicken thighs to the marinade in the Ziploc bag. Seal the bag, making sure the chicken is coated evenly by the marinade. Refrigerate for at least 4 hours or preferably overnight.
After at least 4 hours or overnight, remove the chicken from the fridge and let it sit on the counter for about 20-30 minutes. Meanwhile, get a grill preheated. Sprinkle some brown sugar over pineapple slices, and grill on both sides until they get nice grill marks. Set aside.
Cook the chicken: Get the grill preheated and brush with some olive oil. Discard marinade and grill chicken thighs for about 6-8 minutes per side or until cooked through. During the last 2 minutes, brush with reserved marinade. Tip: Sometimes chicken thighs can be hard to cook through properly depending on how thick they are. So, if needed, finish cooking the chicken in a 400F oven for 10-12 minutes.
Serve the chicken with the grilled pineapple and garnish with green onions. Enjoy!