Preheat and prep: Heat your oven to 350°F (175°C).
Line a baking sheet with foil or lightly grease a loaf pan. If using a loaf pan, sprinkle 1 to 2 tablespoons of brown sugar on the bottom for extra caramelization.
Make the glaze: In a small bowl, mix 1/3 cup brown sugar, ketchup, 1 tablespoon Worcestershire sauce, Dijon, and vinegar. Set aside.
Soften the aromatics: If you prefer a milder onion bite, sauté the minced onion and garlic in a teaspoon of oil for 3 to 4 minutes until soft.
Cool slightly.
Build the base: In a large bowl, whisk eggs, milk, remaining 3 tablespoons ketchup (if using), Worcestershire, salt, pepper, and paprika. Stir in breadcrumbs to hydrate.
Add the beef: Gently fold in the ground beef and onions until just combined. Do not overmix or the meatloaf can turn dense.
Shape the loaf: Transfer the mixture to your pan or baking sheet and shape it into a loaf about 8 to 9 inches long.
Create a shallow trench on top so the glaze stays put.
Glaze generously: Spoon half the glaze over the top and sides. Reserve the rest for later.
Bake: Place in the oven and bake for 45 minutes. Remove, brush with remaining glaze, and return to the oven.
Finish cooking: Continue baking 15 to 25 minutes, or until the internal temperature reaches 160°F (71°C).
Total bake time is typically 60 to 70 minutes, depending on thickness and your oven.
Rest and slice: Let the meatloaf rest for 10 minutes to hold its shape. Slice with a sharp knife and serve warm.