Preheat your oven to 350°F (175°C). Line a 9x5 inch loaf pan with parchment paper for easier removal, or lightly grease the pan.
Process the oats (optional). For a finer texture, pulse the oats a few times in a food processor. This step is optional but creates a more traditional meatloaf texture.
Combine all meatloaf ingredients. In a large bowl, gently mix the ground beef, oats, eggs, onion, garlic, 1/4 cup of the ketchup, Worcestershire sauce, Dijon mustard, thyme, salt, pepper, and nutmeg if using. Mix with your hands until just combined—don't overmix or your meatloaf will be tough.
Perform a taste test. Take a small portion of the mixture and quickly cook it in a microwave or small skillet. Taste and adjust seasonings if needed.
Form your loaf. Transfer the meat mixture to the prepared loaf pan and shape into a loaf, pressing down gently to remove air pockets.
Make the glaze. In a small bowl, whisk together the remaining ketchup, brown sugar, apple cider vinegar, and Worcestershire sauce until well combined.
Apply half the glaze. Spread half of the glaze mixture evenly over the top of the meatloaf.
Begin baking. Place the meatloaf in the preheated oven and bake for 30 minutes.
Add remaining glaze. After 30 minutes, remove the meatloaf from the oven and carefully drain off any excess fat if desired. Spread the remaining glaze over the top.
Finish baking. Return the meatloaf to the oven and bake for an additional 25-30 minutes, or until an instant-read thermometer inserted into the center registers 160°F (71°C).
Rest before slicing. Remove from the oven and let the meatloaf rest for 10-15 minutes before slicing. This step is crucial for a meatloaf that holds its shape when cut.
Slice and serve. Carefully lift the meatloaf from the pan using the parchment paper overhang if used. Slice into thick pieces and serve hot.