In a small bowl, prepare the Parmesan-thyme breading by combining 1/2 cup panko bread crumbs, 1/4 cup grated Parmesan cheese, 4 tablespoons melted unsalted butter, 1 teaspoon minced fresh thyme leaves, pinch of salt and pepper, and mix together until well incorporated. Set aside.
Preheat oven to 400 degrees Fahrenheit
In a cold medium-sized saucepan, add olive oil and bacon, bring to medium heat and sauté for about 6 minutes until bacon is browned.
Remove bacon from pan, reserving bacon fat and add sliced garlic. Sauté for about 35 to 40 seconds until garlic is slightly browned and tender (careful not to burn), then remove garlic from bacon fat.
Grease the inside of a 10” cast-iron skillet with the duck fat or butter.
In a large bowl, toss sliced potatoes with 1 tablespoon kosher salt and 1 teaspoon black pepper until well-coated. Let sit for 10 minutes.
Create a thin layer of potatoes on the bottom of the pan, following the circular shape of the pan, overlapping each potato 1/2”.
Evenly pour 1/4 cup heavy cream over the potatoes. Then sprinkle 1/3 of the bacon, 1/3 of the garlic, 1/4 cup parmesan cheese, and a little black pepper to taste. Repeat this process 2 more times.
Create a 4th layer of potatoes, top with remaining heavy cream, remaining Parmesan cheese, minced thyme leaves and sprinkle black pepper. Loosely cover with aluminum foil and bake on the middle rack of the oven for 1 hour.
After 1 hour, remove aluminum foil and bake an additional 20 to 25 minutes until top layer is bubbly and golden brown.
Remove from oven and evenly spread Parmesan-thyme breading over the top of au gratin potatoes, and place back in the oven.
Broil until the top is golden brown, about 3 to 5 minutes, rotating the pan as needed. (Carefully watch to not burn.)
Remove from oven and let potatoes rest 10 minutes before slicing. Resting the au gratin allows the potatoes to set and keep their shape.