Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x5-inch loaf pan or line a baking sheet with parchment paper for a free-form loaf.
Step 2: In a large mixing bowl, combine the beaten egg, milk, and plain breadcrumbs. Let this mixture sit for 5 minutes to allow the breadcrumbs to fully absorb the liquid.
Step 3: Add the ground beef, finely chopped onion, minced garlic, chopped fresh parsley, salt, black pepper, and dried thyme to the breadcrumb mixture.
Step 4: Using your hands, gently mix all the ingredients until just combined. Be careful not to overmix, as this can lead to a dense, tough meatloaf.
Step 5: Transfer the meat mixture to the prepared loaf pan and gently press it evenly, or form it into a loaf shape on the parchment-lined baking sheet.
Step 6: In a small bowl, whisk together all the glaze ingredients: ketchup, brown sugar, apple cider vinegar, and Worcestershire sauce. Spread about half of this glaze evenly over the top of the meatloaf.
Step 7: Bake for 45 minutes. After 45 minutes, remove the meatloaf from the oven and spread the remaining glaze over the top.
Step 8: Return the meatloaf to the oven and bake for another 30-40 minutes, or until the internal temperature reaches 160°F (71°C) when measured with a meat thermometer inserted into the thickest part of the loaf.
Step 9: Once baked, remove the meatloaf from the oven and let it rest in the pan for 10-15 minutes before slicing.
Step 10: Carefully drain any excess fat from the pan before transferring the meatloaf to a cutting board. Slice into thick pieces and serve warm.