In a large mixing bowl, combine the ground beef, fine breadcrumbs, whole milk, minced yellow onion, beaten eggs, coarse salt, and black pepper. Use your hands to gently mix until just combined, taking care not to overwork the meat.
Shape the mixture into a uniform, compact oblong loaf that fits comfortably inside your slow cooker insert. Place the shaped loaf directly into the center of the cold pot.
In a separate small bowl or measuring cup, whisk together the heavy cream, dijon mustard, and garlic powder until smooth.
Pour the cream mixture slowly over the top of the meatloaf, using a spoon to spread it evenly so the top is completely coated and some sauce runs down the sides.
Cover the slow cooker with the lid. Cook on Low for 6 to 8 hours, or on High for 4 hours, until the internal temperature of the center reaches 160°F.
Carefully lift the meatloaf out of the pot using two large forks, transfer it to a serving platter, and spoon the remaining rich sauce from the pot over the top before slicing.