For the meatloaf: Preheat the oven to 350 degrees F. Heat a small skillet over medium heat. Add the oil and onions, season with some salt and saute until translucent, 3 to 5 minutes. Add the garlic and saute for 1 to 2 minutes without letting the garlic brown. Remove from the heat and let cool.
In a large bowl, add the crackers, milk, mustard, Worcestershire, hot sauce and eggs. Whisk together, and then sprinkle with salt and pepper. Add the meat, parsley and cooled onions. Mix with your hands until evenly combined. Bake a tester meatloaf patty to taste and test the seasoning, adjusting as necessary. Using your hands, gently form the meatloaf into a loaf about 9-by-5-inches. Place on a baking sheet.
For the glaze: In a small bowl, combine the ketchup, brown sugar and apple cider vinegar. Slather half of the glaze over the meatloaf. Make sure not to touch the meatloaf with the spoon and then put that spoon back into the glaze; don't cross contaminate, people! Bake until the meat registers 160 degrees F on an instant-read thermometer, about 1 hour. Let rest for 20 minutes before slicing for sandwiches.
For the sandwich build: Place a slice of cheese on the bottom griddled bun, then a slice of meatloaf, spoon some extra glaze on the loaf, pop on a few pickles and top with crispy onions. Repeat to make 3 more sandwiches.