Preheat your oven to 375°F (190°C) and line a rimmed baking sheet with foil or parchment paper.

In a small bowl, whisk together the ketchup, brown sugar, apple cider vinegar, and mustard to create the glaze. Set aside.

Place the peeled and cubed potatoes in a large pot of salted water. Bring to a boil and cook for 15-20 minutes until tender.

While potatoes cook, combine the ground beef, breadcrumbs, onion, garlic, eggs, milk, parsley, thyme, salt, pepper, and 1 cup of the shredded cheddar cheese in a large bowl. Mix gently with your hands just until combined—do not overmix.

Shape the meat mixture into a loaf (about 9x5 inches) on the prepared baking sheet. Brush half of the glaze generously over the top and sides.

Bake the meatloaf for 45 minutes. Remove from the oven, brush with the remaining glaze, and bake for another 10-15 minutes, or until the internal temperature reaches 160°F (71°C).

While the meatloaf finishes baking, drain the potatoes and return them to the pot. Add butter and warm milk, then mash until creamy. Stir in salt and pepper to taste.

Let the meatloaf rest for 10-15 minutes before slicing to ensure the juices redistribute. Serve slices with the mashed potatoes.