In a stainless steel pot, bring sour beer or homemade malt vinegar, garlic, pickling spices, onions, bay leaves and salt to a boil and cook for 3 minutes to combine the flavors. Turn off the heat and let rest until marinade is room temperature. This will let the spices bloom into the beer solution, much like a ‘tea’ infusion. Place into either a large seal-able container or Ziploc bag and add the roast. Remove as much air as possible and seal, placing in the refrigerator for 3 – 4 days. If you have the ability and equipment, a vacuum chamber set to high is a great way to increase the flavor and better penetrate the muscle fiber. At least once a day, flip the bag to distribute the marinade equally over the meat.