Whisk together salt, pepper, and next 5 ingredients (through olive oil) in a large mixing bowl. Stir in garlic, onions and bay leaves. Let steep for 1 to 3 hours. Using a ladle, remove about 0.5 cups of marinade, being careful NOT to get any onions. Vacuum seal the marinate with the pork shoulder (you may find it useful to freeze the marinate for 1 to 2 hours). Drop the bag into a 150ºF water bath, and allow to cook for 20 – 30 hours. Refrigerate the remaining onion/marinade mixture.