Start by removing any loose or bruised outer leaves from your cabbage. Place the head on a cutting board and cut it into thick wedges, making sure to keep a piece of the core attached to each wedge so they don’t fall apart.
Place a large cast-iron skillet over medium-high heat and add two tablespoons of butter. Once the butter is bubbling, lay the cabbage wedges into the pan in a single layer. Let them cook undisturbed for about four to five minutes until the bottom side is deeply browned and caramelized. Use tongs to carefully flip them over and sear the other side.
Grab a small bowl or a measuring jug to prepare your sauce. Whisk together the heavy cream, minced garlic, dried thyme, salt, and black pepper. If you want a slightly lighter version, you can use half-and-half
If your skillet has enough room, push the cabbage wedges to the side and add the remaining tablespoon of butter and the minced garlic. Let the garlic cook for just thirty seconds until it becomes fragrant.
Turn the heat down to medium and pour the cream mixture all over the seared cabbage wedges. Use a spoon to make sure the liquid gets down into the cracks of the cabbage layers. Let the liquid come to a very gentle simmer on the stove for about two minutes.
Top the cabbage wedges with a generous layer of shredded Gruyère and Parmesan cheese. In a small bowl, toss the Panko breadcrumbs with two tablespoons of olive oil until they are evenly coated. Sprinkle the breadcrumbs over the top of the cheese.
Slide the entire skillet into an oven preheated to 400°F. Bake the gratin for about 20 to 25 minutes. You can test the doneness by sticking a knife into the thickest part of a cabbage wedge; it should slide in with almost no resistance.
