Prepare hot coals or embers for Dutch oven cooking. You'll need to start with 24 coals. They're ready to use when they're glowing and covered in white ash.
In a large bowl, combine the bread crumbs and the milk and stir together. Let soak for 5 minutes.
Add the beef, egg, onion, Worcestershire sauce, salt, and pepper and use your hands to mix everything thoroughly.
Shape the mixture into a 5-by-7-inch rectangle (about 1 ½ inches tall) on a piece of parchment paper. Use the sides of the parchment to lift the meatloaf into the Dutch oven, then cover with the lid.
Place the Dutch oven in the fire pit and arrange 8 coals below it and 16 on the lid. Cook for 30 minutes. After 15 minutes, begin preparing another batch of coals.
Make the glaze
While the meatloaf is cooking, mix the ketchup, mustard, and Worcestershire sauce together in a medium bowl.
After the meatloaf has cooked for 30 minutes, uncover it and brush it all over with the glaze.
Cover the Dutch oven and arrange a new batch of hot coals (8 on the bottom and 16 on the lid). Cook until the internal temperature of the meatloaf reaches 165°F (74°C) on an instant read thermometer, 15 to 20 minutes more.
Remove the Dutch oven from the heat. Use the parchment to remove the meatloaf from the pot and let it rest for 10 minutes before slicing and serving.
Notes
Protect your Dutch oven--Since you're cooking in a Dutch oven outside, the cooking time can vary due to outside temperature and wind. If you're cooking on a very windy day, try to protect the Dutch oven with a barrier, such as a tarp.
Let it rest--Don't be tempted to slice into your meatloaf immediately. It will fall apart! Let it rest for 10 minutes before cutting and you'll be rewarded with nice even slices.
Storage and reheating--Slice any leftover meatloaf and store it in a sealed container in your fridge or cooler for up to 4 days. Rewarm each slice in a skillet or on a griddle until heated through. It makes a great sandwich filling, or can be added to your loaded hash browns or poutine for breakfast!