Heat the oven to 350°F (175°C). Line a rimmed baking sheet with foil and set a wire rack inside, or use the sheet directly.
Melt the unsalted butter in a skillet over medium heat. Add the finely chopped onion and cook until translucent and softened, 5–7 minutes. Stir in the minced garlic and cook until fragrant, 30–60 seconds. Remove from heat and let cool 5 minutes.
In a large bowl, combine the milk and plain dried breadcrumbs. Let stand until the breadcrumbs absorb the liquid and form a thick panade, 5 minutes.
Add the eggs, Worcestershire sauce, kosher salt, black pepper, fresh parsley, and the cooled onion–garlic mixture to the bowl. Mix until a smooth, cohesive paste forms.
Add the cold ground beef (80/20) and gently mix by hand until just combined with no dry pockets, 30–60 seconds. Do not overwork.
Transfer the mixture to the prepared baking sheet and shape into a 9Ă—5-inch loaf with a slight dome and smooth sides.
In a small bowl, whisk the ketchup, brown sugar, apple cider vinegar, and Dijon mustard until smooth to make the glaze.
Brush the loaf with half of the glaze. Bake for 35 minutes.
Brush evenly with the remaining glaze. Continue baking until the center reaches 160°F (71°C) and the glaze is glossy and bubbling at the edges, 20–30 minutes more. For deeper caramelization, broil 1–2 minutes, watching closely.
Rest the meatloaf on the pan for 10–15 minutes to set juices. Slice into 3/4-inch slices and serve warm.