- 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.