Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with butter or non-stick cooking spray.

Heat the olive oil in a large skillet over medium-high heat. Season the chicken pieces with half of the salt and pepper, then sear them for 5 to 6 minutes until golden brown on the outside but not fully cooked through.

In a medium saucepan, combine the heavy cream, chicken broth, minced garlic, and fresh thyme leaves. Bring the mixture to a gentle simmer over medium heat for about 3 minutes until highly fragrant, then remove it from the heat.

Arrange half of the sliced potatoes in an overlapping layer at the bottom of the prepared baking dish. Evenly scatter the seared chicken pieces over the potatoes, then sprinkle with half of the shredded Gruyère cheese.

Layer the remaining sliced potatoes over the chicken and cheese. Carefully pour the warm garlic cream mixture over the entire dish, ensuring all the potatoes are evenly coated.

Cover the baking dish tightly with aluminum foil and bake for 35 minutes. Remove the foil, sprinkle the remaining Gruyère cheese on top, and bake uncovered for another 15 minutes until the cheese is bubbling and deeply golden brown.

Let the gratin rest for 10 minutes at room temperature before serving to allow the rich cream sauce to thicken. Garnish with a few extra fresh thyme leaves and serve alongside a crisp green salad.