Boil potatoes until fork-tender. Let cool. Now Heat a large nonstick skillet on medium heat. Broil bacon until crisp.
Use a slotted spoon to remove the bacon and place it on a plate lined with paper towels to drain. Take out 1 tablespoon. Now Sauté the onions in bacon fat until softened and caramelized. Salt and pepper to taste.
Add the thyme. Allow cooling. In a medium bowl, combine cream, milk, and garlic. Add salt. Set aside. Cut potatoes into 1/4 inch (6mm), slices, and place aside. Grease an oval 13×9 (32cm x23cm) gratin pan.
Now Place one layer of potato slices on a lined pan. After this, Sprinkle Gruyere cheese, chopped bacon, onions, and the milk/cream mix.
After this, Continue this process until you have created three layers. To create a top-and-bottom, slice the cheese wheel horizontally like you would a hamburger bun. Cut the wheel in quarters.
Eight cheese triangles will be created. Now Sprinkle cheese triangles over the top of the casserole. Sprinkle the remaining 1/2 teaspoon of fresh thyme on top. After this, Bake at 400F for 35-40 minutes or until cheese is melted and toppings are golden brown.