Creating this amazing dish is simple. Follow these steps for perfection:

1. Prepare the Slow Cooker: Lightly grease the slow cooker with cooking spray or butter.
2. Slice the Potatoes: Use a mandoline or sharp knife to slice the potatoes into uniform thin rounds for even cooking.
3. Layer the Ingredients: Begin layering one-third of the sliced potatoes in the slow cooker.
4. Add Cheese: Top the potatoes with one-third of the sharp cheddar and Gruyère cheeses.
5. Add Seasonings: Sprinkle some chopped onions, garlic, salt, black pepper, and thyme over the cheese layer.
6. Repeat Layers: Repeat the layering process two more times, finishing with the remaining cheeses on top.
7. Mix Cream and Milk: In a separate bowl, whisk together the heavy cream and milk.
8. Pour Mixture: Pour the cream and milk mixture evenly over the layered potatoes and cheese.
9. Cover and Cook: Seal the slow cooker with the lid. Cook on low for about 4 hours, or on high for about 2 hours, until the potatoes are tender and the cheese is bubbling.
10. Serve: Once cooked, let it sit for about 10 minutes before serving. Garnish with fresh parsley if desired.

By following these straightforward steps, you’ll create a delicious dish that leaves your guests raving!