Preheat the oven. Set it to 350°F (175°C).
Line a sheet pan with foil and lightly oil it, or use a loaf pan lined with parchment for easy removal.
Make the panade. In a large bowl, combine the breadcrumbs and milk. Let it sit for 3–4 minutes until the bread is fully hydrated and soft.
Add aromatics and seasonings. Stir in the egg, grated onion (with juices), garlic, Worcestershire sauce, Dijon, paprika, salt, pepper, thyme, and parsley. Mix until smooth and evenly combined.
Add the meat gently. Add the ground beef and pork.
Using your hands or a fork, lightly mix just until combined. Do not overmix, or the loaf will turn dense.
Shape the loaf. Transfer the mixture to the sheet pan. Shape it into a compact loaf about 9 inches long and 2.5 inches tall. Keep the top slightly domed for even cooking and a good glaze surface.
Mix the glaze. In a small bowl, whisk ketchup, brown sugar, vinegar, and Worcestershire.
Brush half the glaze over the loaf.
Bake, then reglaze. Bake for 35 minutes. Brush on the remaining glaze. Continue baking 15–25 minutes more, until the center reaches 160°F (71°C).
Rest before slicing. Let the meatloaf rest for 10–15 minutes.
This helps juices redistribute so every slice stays moist.
Slice and serve. Use a sharp knife to cut thick slices. Serve with mashed potatoes, green beans, or a crisp salad.