Finely dice the onion into roughly 1/2-inch pieces and measure out all your dry ingredients (salt, garlic powder, black pepper, thyme, paprika, and optional cinnamon) into a small bowl. In a large mixing bowl, combine the ground beef with the dry ingredient mixture, then add the diced onion, egg, breadcrumbs, ketchup, and Dijon mustard. Mix gently with your hands until just combined—overmixing will make the meatloaf dense and tough. I find using my hands is the best way to ensure everything is evenly distributed without compacting the meat too much.

On a piece of aluminum foil or parchment paper, shape the meatloaf mixture into a loaf approximately 9 inches long and 5 inches wide—this size works perfectly for slow cooker cooking and ensures even heat distribution. Wrap the shaped meatloaf tightly with aluminum foil first (this protects it from freezer burn and helps with slow cooking), then wrap it again with plastic wrap for an extra seal. Transfer the wrapped meatloaf into a freezer bag, label it with the date, and freeze for up to 3 months.

Remove the meatloaf from the freezer and place it in the refrigerator to thaw for 12-24 hours before cooking. Once thawed, carefully remove the plastic wrap (the aluminum foil should remain on to protect the meatloaf and retain moisture during the initial slow cooking phase). This gentle thawing method ensures even cooking throughout and prevents the outside from drying out.

Place the aluminum foil-wrapped meatloaf directly into your slow cooker (no need to add liquid—the ketchup and mustard in the mixture plus the trapped steam will keep it moist). Cook on low for 5-6 hours until the meatloaf reaches an internal temperature of 160°F when checked with a meat thermometer. The long, gentle cooking allows the flavors to meld beautifully and keeps the meatloaf tender and juicy.

Carefully remove the meatloaf from the slow cooker and unwrap the aluminum foil. Return the unwrapped meatloaf to the slow cooker, then prepare a glaze by mixing together additional ketchup and Dijon mustard (roughly 1/4 cup ketchup and 1 teaspoon mustard, adjusted to your taste preference). Spread this glaze evenly over the top of the meatloaf and cook on low for an additional 30 minutes, which allows the glaze to caramelize slightly and develop deeper flavor. I like to add the glaze at the end rather than at the start because it prevents it from washing off during the longer cooking time and keeps the flavors bright and vibrant.

Remove the meatloaf from the slow cooker and let it rest for 5-10 minutes before slicing. This resting period allows the juices to redistribute throughout the meat, ensuring each slice stays moist and tender. Slice and serve warm with your favorite sides.