Preheat oven to 425°F. Place one rack at the lowest position and one at the highest position in the oven. If you have a baking steel, pizza stone, or an overturned sheet pan, place on the bottom rack to preheat.
Melt butter in a small saucepan with thyme sprigs and simmer on low for about 5 minutes.
Add 2 tablespoons of thyme butter to the bottom of a quarter sheet pan.
Peel one potato and slice into very thin slices about 1/16 to ⅛ inch thick.
Arrange potato slices in overlapping rows across the pan, brushing lightly with butter and seasoning with salt.
Continue peeling, slicing layering, buttering, and salting one potato at a time until the pan is filled and two layers are formed.
Drizzle remaining butter on top and sprinkle with remaining salt.
Press parchment paper directly agains the layered potatoes and place another sheet pan or tightly sealed foil on top.
Bake on the lowest rack for 30 minutes, preferably on a pizza steel or pizza stone, or sitting on top of an overturned preheated sheet pan.
Remove cover and parchment and bake on the top rack for 10 minutes until golden and crisp. Broil for 1 to 2 minutes for more browning if desired.
Flip onto a cutting board. If more golden brown crisp is desired on this side as well, transfer back to pan and broil for another 1 to 2 minutes.
Garnish with chives and flaky salt, and slice to serve.