Preheat the oven to 375°F (190°C). Grease a loaf pan or line with parchment paper.
Sauté onions and garlic in a skillet over medium heat for 3–4 minutes. Set aside to cool.
In a large bowl, combine ground beef, breadcrumbs, eggs, milk, tomato paste, Worcestershire sauce, thyme, salt, pepper, and the cooked onion/garlic. Mix gently until combined.
Shape the meat mixture into a loaf and place it in the prepared pan.
Bake uncovered for 40 minutes.
While it bakes, whisk together ketchup, brown sugar, and apple cider vinegar to make the glaze.
Spread the glaze over the meatloaf and return it to the oven for another 15–20 minutes, or until the internal temperature reaches 160°F (71°C).
Let rest for 10–15 minutes before slicing and serving.