Combine the ground beef, breadcrumbs, milk, egg, grated onion, allspice, nutmeg, salt, and pepper in a mixing bowl. Mix until evenly blended, then shape the mixture into a loaf.
Place the loaf in the slow cooker and pour ½ cup of the beef broth around it. Cover and cook on low for 6 to 7 hours, until the internal temperature reaches 160°F.
Lift the meatloaf out of the slow cooker and set it aside on a plate. Leave the remaining broth in the pot.
Melt the butter in a skillet over medium heat. Whisk in the flour and cook for one minute. Slowly whisk in the remaining beef broth and the heavy cream. Continue stirring until the gravy thickens.
Slice the meatloaf and serve it with the warm gravy spooned over the top.