Heat a large, shallow skillet over medium high heat. Add the Dr Pepper, brown sugar, onion, balsamic vinegar, seasoned salt, paprika and cardamom. Bring the mixture to a boil and cook until it is reduced by half.

Add the Dr Pepper glaze and the chicken to the slow cooker and cook on low setting for 8 hours. Remove the lid and shred the chicken with about an hour left to cook.

In a mason jar, whisk together the white wine vinegar, sugar and salt and allow to sit for about 5 minutes until the sugar and salt are dissolved. Stir in the onions and allow to sit on the counter, stirring occasionally for one hour. Store, covered, in the refrigerator until ready to use.

Pile the Dr Pepper glazed chicken onto slider buns and top with the pickled onions to serve.