Line your slow cooker with foil and lightly spray it with cooking oil—this makes cleanup effortless and prevents sticking. In a medium bowl, whisk together the ketchup, brown sugar, grated onion, mustard, and vinegar until smooth and well combined. This glaze will be used in two stages: some mixed into the meatloaf for moisture and flavor, and the remainder as a coating. Set aside 2 tablespoons of glaze for the meat mixture and reserve the remaining glaze for cooking and finishing.

In a large bowl, crumble the ground beef and add the finely diced onion, stuffing mix, milk, eggs, 2 tablespoons of the glaze from Step 1, salt, and pepper. Using your hands or a large spoon, gently mix until just combined—don't overwork the meat, as this makes the meatloaf dense and tough. I find that mixing with my hands but stopping as soon as everything is evenly distributed gives the best tender texture. The mixture should feel moist but hold together when formed.

Gently shape the meatloaf mixture from Step 2 into a compact oval that will fit comfortably in your slow cooker, leaving about 1 inch of space on the sides for heat circulation. Place the shaped meatloaf onto the foil-lined slow cooker. Pour approximately 1/2 cup of the remaining glaze over the top, using a spatula to spread it evenly. Cover the slow cooker and cook on LOW for 6 hours. The low and slow method ensures the meatloaf stays incredibly moist and develops deep, rich flavors.

After 6 hours, carefully transfer the meatloaf to a foil-lined baking sheet using the foil lining as a sling—this prevents it from falling apart. Preheat your broiler to high. Spread the remaining glaze (about 2/3 cup) generously over the top of the meatloaf. Broil for 3–5 minutes, watching carefully, until the glaze is bubbly and slightly caramelized at the edges. This finishing step adds a beautiful glaze and slight char that elevates the flavor.

Let the meatloaf rest for 5 minutes before slicing—this allows the juices to redistribute throughout the meat, keeping each slice moist and tender. Slice into thick portions and transfer to a serving platter. Drizzle with any extra glaze from the baking sheet and serve warm with additional glaze on the side for those who want more.