Lay out the beef rounds in a single layer. Season the meat on both sides with the garlic salt and black pepper. Put the meat in a medium mixing bowl and add the vinegar. Let the meat marinate for at least 2 hours or overnight in the refrigerator.
Preheat a grill to high heat.
Remove the meat from the marinade and put on the hot grill. Grill until the meat is cooked to desired doneness. Transfer the meat to a cutting board. Cut the meat into 1-inch pieces and set aside in bowl, covered with foil, to keep warm.
Heat a large pan or griddle over medium heat. Spread the cut sides of the bolillo rolls with mayonnaise. Toast them in the pan, mayonnaise side down, until golden.
Combine the chopped onions and cilantro in a small bowl.
To assemble the tortas, top the bottom half of the roll with sliced avocado, a handful of the cut meat, some of the cilantro/onion mixture and some of the Torta Salsa. Top with shredded lettuce and 2 tomato slices. Repeat with remaining rolls and ingredients. Cover with the roll tops, cut in half and serve.
Add all of the ingredients to a medium saucepan and bring to boil over medium heat. Cook until the onions are translucent. Remove from the heat and let cool.
Transfer the cooled mixture to a blender and blend until smooth.