Line an 8x4 inch loaf pan with parchment paper, allowing the paper to overhang the sides for easy unmolding.

Using a mandoline slicer set to 1/8 inch thickness or a sharp knife, slice the potatoes into thin even rounds and keep submerged in cold water.

In a medium saucepan over medium heat, combine heavy cream, milk, 4 tablespoons butter, thyme sprigs, salt, pepper, and nutmeg and heat until small bubbles form around the edges, about 4 minutes.

Drain the potatoes and pat dry thoroughly with kitchen towels to remove excess moisture.

Pour a thin layer of the warm cream mixture into the prepared pan, then layer potato slices in slightly overlapping rows and drizzle more cream between layers until all ingredients are used.

Cover the stack with parchment paper and place a heavy weight on top; refrigerate for at least 2 hours, preferably overnight, to firm up.

Invert the chilled pave onto a cutting board using the parchment overhang, remove the paper, and slice into four equal portions with a sharp knife.

Heat a cast iron skillet over medium-high heat, add remaining 2 tablespoons butter, and sear the potato portions flat-side down until deep golden brown, 3 to 4 minutes per side.