Line a rimmed baking sheet with aluminum foil and coat lightly with cooking spray. Heat oil in a skillet over medium heat until shimmering. Cook the onion until golden, about 5-7 minutes. Add the garlic and cook until fragrant, about 30 seconds. Transfer to a large bowl.

Process the saltines and milk in a food processor until smooth. Add the beef and pork and pulse until well combined, about ten 1-second pulses. Transfer the meat mixture to the bowl with the cooled onion mixture. Add the eggs and yolk, mustard, Worcestershire sauce, thyme, parsley, 1 tsp salt and 3/4 tsp pepper to the bowl and mix with your hands until combined.

Adjust the oven racks to the upper (about 4-inches away from broiler elements) and middle positions and heat the broiler. Transfer the meat mixture to the prepared baking sheet and shape it into a 9- by 5-inch loaf. Broil on the upper rack until well browned, about 5 minutes. Brush 2 Tbsp of the reserved, uncooked glaze over the top and sides of the loaf and then return to the oven and broil for another 2 minutes.

Transfer the meatloaf to the middle rack and brush with the remaining uncooked glaze. Change the oven temperature to bake at 350 degrees and bake the meatloaf for 40 to 45 minutes, until cooked through. Transfer the meatloaf to a cutting board and tent with foil. Let it rest for 20 minutes. Slice and serve, passing the cooked glaze at the table.