First, take the reserved liquid and reduce by half and set aside.
In a heavy saucepan, melt the butter. Sauté the onion and garlic in the butter until translucent, about 10 minutes.
Add in the reduced Dr Pepper and reduce again by half.
Add in the rest of the ingredients and simmer for about 15 minutes.
Continue cooking for 20 to 30 minutes, the sauce will begin to thicken.
Let cool. Once cool, put in a blender and blend until smooth.
Shred the beef and put in a pot with about 1/2 cup of the BBQ sauce and 1/2 cup of chicken stock. Simmer on low heat for an hour.
Assemble sandwich with bread, pickles and red onions and be sure to add some more BBQ sauce.