1. Prepare a 9" x 5" loaf pan by lining the bottom with parchment paper.
2. If desired, line a quarter sheet pan with heavy duty foil. This helps with the cleanup but has no effect on the actual cooking so you don't need to do this if you don't want to or if you don't have heavy duty foil.
3. Add the beef, onion, bread crumbs, parsley if you are using it, salt, celery seed, garlic, thyme to a bowl and mix lightly by tossing the ingerdients together to mix them. Do not squeeze the meat together at this point.
4. In a separate bowl, add the eggs and beat lightly. Then add the milk and ketchup and mix.
5. Add the wet ingredients to the meat mixture and mix thoroughly with your hands. It really helps to have disposable gloves here but they are not required. Your hands are the best mixing tools in the kitchen!
6. Add the mixture to the loaf pan and lightly pack it in to make sure there are no air spaces. You don't want to compress the meat mixture too much or it will be too dense to cook properly. Smooth the top so it is even and flat. If you do not have a loaf pan, you can just mold the mixture free-form to make a loaf on a sheet pan. It will work just fine doing it this way.
7. I like to refrigerate it at this point to allow the ingredients a chance to get to know each other. This is not required but you can do it for up to several hours if you want. Just make sure it is covered and refrigerated the whole time. Take it out about 30 minutes before it is going to go in the oven.
8. To make the glaze, add the ketchup, the barbecue sauce and the brown sugar to a small bowl, mix thoroughly and set aside.
9. Preheat the oven to 400° F.
10. Spread about 2 tablespoons of the glaze over the top of the meatloaf with a butter knife or a basting brush.
11. Place the meatloaf in the preheated oven and immediately reduce the temperature to 350° F.
12. Cook until the internal temperature is 155 degrees, about 1 hour to 1 hour and 15 minutes in total. Check it with an instant read thermometer. I prefer not to use a meat thermometer that you insert into the meat loaf because if the very middle is cooked to 155° it will be overcooked. The meat loaf will continue to cook after you take it out of the oven.
13. Remove from the oven, cover loosely with foil and let rest for 10-15 minutes before serving.