1. In a large saucepan over medium-high heat, heat olive oil. Add in sweet onion and saute for about 5 minutes. Add in garlic and saute for another 60 seconds. Stir in raspberries, basil, chipotle chiles, ketchup, brown sugar, Worcestershire sauce, ground mustard, and pepper. Increase heat to high and bring to a boil. Reduce heat to medium-low and simmer for 10 minutes.
2. Transfer sauce to a blender or food processor and blend until smooth. Divide sauce in half and set aside one half in the refrigerator to be used at serving time.
3. Cut rib racks in half. Layer ribs in a crockpot, pouring a little of the remaining sauce over each layer.
4. Cover and cook on low 8 hours.
5. Before serving, lay the racks on an aluminum foil-lined baking sheet. Brush generously with reserved sauce. Broil under a broiler until sauce begins to bubble and caramelize. Serve hot.