Pre-heat your oven to 400 degrees.
In a medium saucepan, melt 1½ tablespoons of butter over medium heat. When the butter is melted and frothy, add the onion and 4 minced garlic cloves and cook for 3-4 minutes until the onion is soft and translucent. (Be very careful not to burn the garlic, peeps!) Add the milk, cream, thyme, rosemary, and a generous amount of salt to the saucepan and bring to a gentle simmer. Reduce the heat to low and simmer very gently for 15 minutes to allow all those delicious flavors to mingle.
Meanwhile, prep the potatoes. Peel the potatoes and slice them very thinly into 1/8-inch discs. If you have a mandolin, you DEFINITELY want to use it (carefully) here. Briefly set aside.
Remove the thyme and rosemary sprigs from the cream mixture and discard. (Some of the rosemary and thyme will have detached from the sprigs. Leave those floaters!) Taste a little bit of the mixture. Add a little extra salt if needed—you want this to be on the salty side—and season with fresh ground pepper.
Pour ½ cup of the cream mixture into the bottom of a 7x11 baking dish. (You can also use a 10- or 12-inch cast iron skillet if you don’t have a baking dish.) Add one third of the potatoes in an even layer. Pour another ½ cup of the cream mixture over the potatoes and sprinkle with half of the crumbled goat cheese. Top with a second layer of potatoes, followed by ½ cup cream, and the remaining goat cheese.Add the third and final layer of potatoes, and pour the remaining cream mixture over the top of your gratin.
Cover the baking dish tightly with foil and bake for 50 minutes until the potatoes are very tender when pierced with the tip of a sharp knife.
While the gratin is in the oven, make your crunchy topping. Heat the remaining 1 tablespoon of butter in small skillet. When melted, add the remaining minced garlic clove and cook for 30-45 seconds, just until fragrant. (Don’t burn it!!) Stir in the panko and a pinch of salt and cook, stirring regularly until the panko darkens a shade in color. Transfer the topping to a bowl to cool. When cool, mix in the grated Parmesan.
Remove the foil from the gratin. (It will look like there’s too much liquid in your gratin. Don’t panic, it’s just the right amount.) Add the panko topping in an even layer. Return the gratin to the oven for another 15 minutes until the top is crispy and browned and some of the liquid has been absorbed.
Allow the gratin to rest for at least 10 minutes before slicing and serving.