Preheat your oven to 350°F (175°C). Line a baking sheet with foil (if using) or lightly grease a 9x5 inch loaf pan.
Sauté the aromatics. This step is optional but recommended: In a small skillet over medium heat, sauté the onions in a bit of oil until translucent, about 5 minutes. Add the garlic and cook for 30 seconds more. Allow to cool slightly. (This removes the raw onion bite and adds more flavor.)
Soak the oats. In a small bowl, combine the oats and milk. Let sit for 5 minutes to soften. This is your secret weapon for moisture.
Combine all ingredients. In a large bowl, gently mix the ground beef, ground pork (if using), beaten eggs, sautéed onions and garlic, soaked oats with milk, ketchup, Worcestershire sauce, Dijon mustard, parsley, thyme, salt, pepper, and nutmeg. Mix with your hands just until combined—don't overwork it.
Perform the test. Take a small portion of the mixture and microwave it for 20-30 seconds or cook it in a small skillet. Taste for seasoning and adjust salt and pepper if needed.
Shape your loaf. Transfer the meat mixture to your prepared loaf pan and shape into a loaf. If using a baking sheet, form the mixture into a loaf shape about 9x5 inches.
Make the topping. In a small bowl, mix together the ketchup, brown sugar, apple cider vinegar, and Worcestershire sauce until the sugar dissolves.
First bake. Bake the meatloaf uncovered for 40 minutes.
Add the topping. After 40 minutes, remove the meatloaf from the oven and carefully drain off any excess fat. Spread the topping mixture evenly over the meatloaf.
Finish baking. Return to the oven and bake for an additional 30-35 minutes, or until an instant-read thermometer inserted into the center registers 160°F (71°C).
Rest before slicing. Allow the meatloaf to rest for 10-15 minutes before slicing. This helps it hold together better and keeps the juices from running out.