Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper.
For the meatloaf: Heat a skillet over medium heat, add the oil and saute the onions until translucent, 3 to 5 minutes. Season with salt, add the garlic and saute for 1 to 2 minutes without letting the garlic brown. Set aside to cool.
In a large bowl, whisk together the milk, mustard, Worcestershire, hot sauce and eggs, then season with salt and pepper. Add the beef, pork and veal along with the crackers, parsley and cooked onion-garlic mixture. Use your hands to mix until everything is evenly combined. Place the meat mixture on the prepared baking sheet and use your hands to gently form into a rustic loaf shape, about 9 inches by 5 inches.
For the glaze: In a small bowl, combine the barbecue sauce, brown sugar, apple cider vinegar and sriracha.
Slather the glaze all over the meatloaf, then bake, basting once halfway through, until the internal temperature registers 165 degrees F, about 1 hour. Let rest for 20 minutes before serving.
Before baking the whole meatloaf, bake a small tester patty of the mixture to test for seasoning.