Prepare the bottom and sides of a slow cooker with nonstick cooking spray.

In a large skillet over medium heat, cook ground beef with the onions until the beef is browned and fully cooked. Drain any excess fat.

To the skillet, add the garlic and seasonings. Stir to combine.

In a small saucepan over medium heat, melt butter. Stir in flour and cook for about 1 minute, making a roux.

Slowly whisk in beef broth and half and half into the roux, stirring constantly until the mixture thickens.

Create a layer of the potatoes in the bottom of the slow cooker.

Sprinkle a layer of the ground beef and onion mixture over the potatoes.

Top the ground beef with a sprinkle of each of the cheeses.

Repeat the layering process with the remaining potatoes, ground beef and onion mixture, and cheeses, ending with a layer of cheese.

Pour the creamy sauce over the layered ingredients in the slow cooker, ensuring that it covers everything evenly.

Cover the slow cooker and cook on low heat for 4 to 5 hours or until the potatoes are tender and the sauce is bubbly and thickened.

Once the cooking time is complete, carefully remove the lid, and let the au gratin sit for a few minutes to settle.

Garnish with chopped fresh parsley if desired.