Preheat your oven to 350°F (175°C). Line a baking sheet with foil or lightly grease a 9x5 inch loaf pan.
Sauté the aromatics (optional but recommended). In a small skillet, heat a bit of oil and sauté the diced onion until translucent, about 5 minutes. Add the garlic and cook for another 30 seconds. Let cool slightly. (This step is optional but helps eliminate that raw onion bite and adds more flavor.)
Mix the meat mixture. In a large bowl, combine the ground beef, cooled onion and garlic mixture, eggs, breadcrumbs, ketchup, Worcestershire sauce, Dijon mustard, thyme, salt, black pepper, cayenne (if using), and parsley. Mix with your hands until just combined—don't overmix or your meatloaf will be tough.
Shape your loaf. Transfer the meat mixture to the prepared loaf pan and shape into a loaf, or form it into a loaf shape (about 9x5 inches) on the lined baking sheet. Using the baking sheet method allows more surface area for the glaze but either way works great.
Make the glaze. In a small bowl, whisk together the BBQ sauce, brown sugar, apple cider vinegar, Worcestershire sauce, and garlic powder until the sugar dissolves.
Initial bake. Bake the meatloaf for 30 minutes uncovered.
Apply the glaze. After 30 minutes, remove the meatloaf from the oven and carefully pour off any excess fat. Brush half of the glaze evenly over the top and sides of the meatloaf.
Continue baking. Return the meatloaf to the oven and bake for an additional 20 minutes.
Final glaze and finish. Brush the remaining glaze over the meatloaf and bake for 10 more minutes, or until the internal temperature reaches 160°F (71°C) on a meat thermometer.
Rest before slicing. Allow the meatloaf to rest for 10 minutes before slicing. This helps it hold together better when serving.