Preheat an oven. Place peeled potatoes in a large mixing bowl. Set a 10-inch cast-iron skillet over medium heat, then add unsalted butter. Once melted, add smashed garlic, Kosher salt, and black pepper, then stir well. Pour over the potato slices and toss to evenly coat.
Arrange the potato slices in a concentric overlapping circle. Once you have the base layer, sprinkle the grated Parmesan cheese. Repeat with remaining potatoes and Parmesan cheese.
Transfer to the oven and cook until the potatoes are crispy and golden brown, about 50-60 minutes. Cool slightly before flipping.
Combine creme fraiche in a bowl with chopped dill, and fresh lemon juice. Stir well, then refrigerate until needed. The sauce will firm up in the fridge.
Place an inverted plate on top of the skillet that’s slightly larger than the pan. Carefully invert the two and flip over. The potato galette should easily release onto the plate. Sprinkle with a generous pinch of flaky sea salt.
Spoon the creme fraiche sauce directly on top of the galette, then add sliced salmon. I like to fold the smoked salmon onto itself a little to make a pretty presentation. Garnish with chopped red onion, capers, and chives, then enjoy immediately!