Slow Cooker Root Beer BBQ Pork Ribs

Name: Slow Cooker Root Beer BBQ Pork Ribs
Ingredients: 2 racks pork baby back ribs (membrane removed); 1 large onion (thinly sliced); 1.5 cups root beer (divided); 3 tbsp kosher salt; 1 tbsp dried oregano; 1 tbsp garlic powder; 1 tbsp black pepper; 1 tbsp chili powder; 1 tbsp smoked paprika; 1 tbsp olive oil; 1.5 cups onion (finely chopped); 1 tbsp garlic (minced); 1.5 cups BBQ sauce (bottled or homemade); 0.5 cup apple cider vinegar; 0.25 cup brown sugar; 1 tbsp chipotle in adobo sauce (minced); 1 tsp Worcestershire sauce; salt & pepper (to taste)