In a mixing bowl, combine the finely chopped onion, minced garlic, ketchup, orange marmalade, crushed pineapple, honey, Worcestershire sauce, brown mustard, salt, black pepper, and sriracha (if using); stir until smooth and evenly mixed.
Place the skin-removed bone-in chicken pieces in the slow cooker in a single layer or slightly overlapping.
Pour the prepared pineapple-orange barbecue sauce over the chicken, turning pieces to coat if needed.
Cook on LOW for 4–6 hours or on HIGH for 3–4 hours, until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).
Carefully transfer the cooking liquid from the slow cooker to a saucepan and bring it to a simmer over medium heat.
Whisk the cornstarch slurry (1/4 cup cornstarch dissolved in 1/4 cup water) into the simmering sauce and whisk continuously until the sauce thickens, about 1–2 minutes.
Pour the thickened sauce back over the chicken in the slow cooker or serving dish and toss or spoon to coat before serving.
Serve the chicken with rice, mashed potatoes, or your choice of vegetables.