Preheat the oven to 350°F (175°C). Lightly grease a loaf pan or line a baking sheet.
Make the glaze: whisk together Ketchup, Brown Sugar, Apple Cider Vinegar, Worcestershire Sauce, a pinch of Kosher Salt and freshly ground Black Pepper; set aside.
Heat a tablespoon of oil or butter in a skillet over medium heat and sauté the Onion and Garlic until soft and translucent, about 4–5 minutes; remove from heat and let cool slightly.
In a large bowl combine the Ground Beef with Breadcrumbs, Milk, Eggs, the cooled onion and garlic, and 1–2 tablespoons of the reserved glaze (or a splash of Worcestershire). Season lightly with salt and pepper and mix gently until just combined—overmixing will make the loaf dense.
Shape the mixture into a loaf and transfer to the prepared pan. Spread about half of the glaze evenly over the top of the loaf.
Bake for 50–60 minutes, until an instant-read thermometer inserted into the center reads 160°F (71°C). In the last 10 minutes of baking, brush on the remaining glaze and return to the oven to caramelize.
While the meatloaf bakes, prepare the mashed potatoes: peel and cut the Russet Potatoes into even chunks, cover with cold water, add a pinch of salt, bring to a boil, then simmer until very tender, about 15–20 minutes.
Drain the potatoes well, return them to the pot, add Butter, a splash of Milk and Heavy Cream, then mash to your desired consistency. Taste and adjust seasoning with salt and pepper.
When the meatloaf is done, let it rest 8–10 minutes before slicing to keep it juicy. Serve slices of meatloaf with a generous scoop of mashed potatoes and spoon any pan juices or extra glaze over the top.
