Combine pineapple juice, brown sugar, ketchup, soy sauce, garlic, worcestershire sauce, and spices in a large measuring cup or mixing bowl. Mix until all ingredients are incorporated; set aside.
Add pineapple juice mixture to the skillet with the butter, onion and flour. Bring to a boil.
Boil, stirring, 10-15 minutes or until sauce is thick.
Based on readers' comments, I updated the recipe on 8/11/17 by increasing the cooking time on the sauce prior to adding it to the slow cooker.