Preheat your oven to 350°F (175°C). Line a 9x5 inch loaf pan with parchment paper, leaving some overhang for easier removal, or lightly grease with cooking spray.
Prepare the crushed corn flakes. Place corn flakes in a zip-top bag and crush them with a rolling pin, or pulse briefly in a food processor. They should be fine but not powdery.
Soak the corn flakes. In a medium bowl, combine the crushed corn flakes and milk. Let them soak for 5 minutes until softened. This creates that perfect texture unique to grandma's recipe.
Mix all ingredients. In a large bowl, combine the ground beef, soaked corn flakes with any remaining milk, diced onion, diced bell pepper, beaten eggs, ketchup, Worcestershire sauce, mustard, salt, pepper, garlic powder, and thyme. Mix gently with your hands until just combined—don't overmix or your meatloaf will be tough.
Form your loaf. Transfer the meat mixture to the prepared loaf pan, pressing down gently to remove air pockets. Shape the top into a slight dome.
Make the topping. In a small bowl, whisk together the ketchup, brown sugar, apple cider vinegar, and Worcestershire sauce until smooth.
Top the meatloaf. Spread the topping mixture evenly over the meatloaf.
Cover and bake initially. Cover the loaf pan loosely with aluminum foil to prevent the topping from burning. Place in the preheated oven and bake for 45 minutes.
Finish uncovered. Remove the foil and continue baking for an additional 25-30 minutes, or until the internal temperature reaches 160°F (71°C) and the topping is caramelized.
Rest before slicing. Allow the meatloaf to rest in the pan for 15 minutes before removing and slicing. This step is crucial for a meatloaf that holds together beautifully when sliced.
Slice and serve. Using the parchment paper overhang, carefully lift the meatloaf from the pan, transfer to a cutting board, and slice into thick pieces. Serve hot with classic sides like mashed potatoes and green beans.