Combine meat, egg, onion, Worcestershire sauce, milk, and breadcrumbs in a large mixing bowl. Use your hands to thoroughly mix the ingredients until the mixture is uniform.
In a separate bowl, mix ketchup, mustard, and brown sugar together.
Package meatloaf mixture in a quart sized resealable freezer bag. Be sure to remove as much air from the bag as possible. Using a Sharpie, label the bag with the contents and date.
Add the ketchup glaze to a snack sized resealable bag. Staple the glaze to the top of the meatloaf bag, being careful not to puncture through the portion of the bag containing the contents. Place in the freezer.
When ready to use, thaw meatloaf on counter for approximately 8 hours or in the fridge for 24 hours.
Heat the oven to 350 degrees. Spray your desired pan with grease. I use either a bread loaf pan or an 8 x 8 or 9 x 9 inch pan. Put the meat into the pan, pressing firmly with your hands. Add the glaze to the top, using the back of a spoon to spread it evenly.
Bake for 1 hour or until a meat thermometer indicates a temperature higher than 165 degrees. If you don't have a meat thermometer, you can make a cut into the center of the loaf and make sure the meat is no longer pink. Another indication that it is done is that the juices will be clear.
Whenever I make this, I have a substantial amount of liquid surrounding the edges of the meatloaf. I use two spatulas to carefully remove it from the pan and place it on a plate, so it's easier to cut.