Combine the beef, pork, club soda, salt, garlic, pepper, baking soda and bread in a mixing bowl and mix together by hand for at least 5 to 10 minutes. Refrigerate, covered, for a few hours or up to 12 hours.
Grab a small handful of the meat mixture and roll between your hands to form breakfast sausage-size links. Repeat with the remaining meat.
Heat a griddle until hot. Lightly grease the griddle with some oil, and then cook the cevapcici, rolling them around occasionally, until cooked though and golden.
Grill the cut sides of the rolls. Sandwich the cevapcici and onions in the rolls with dollops of Kajmak and avjar and serve.
Kajmak (Serbian Cheese Sauce):
Beat together the cream cheese, feta cheese, sour cream and butter if using until fluffy.