Set your mandolin for an 1/8" thick cut, and slice the peeled potatoes. If you don’t have a mandolin, do your best with a sharp knife. Place the slices in a very large bowl. At the sink, fill the bowl up with cold water while tossing the slices around to rinse the starch off. Pour the water out and repeat about four more times until the water stays mostly clear.

Pour off the water and arrange as many slices as will fit on a clean dish towel (you will have to do this in batches). Place another dish towel on top and blot off as much water as you can. Pick up the bottom dish towel and dump the dried slices into a very large bowl. You could use the same bowl you rinsed with, you just have to dump the wet slices out and dry out the bowl. Repeat for the remaining wet slices.

Place the sliced butter in a small Pyrex measuring cup. Microwave it to melt the butter; it’ll take about 30 seconds. Add the cornstarch, thyme, salt, and pepper to the measuring cup and whisk it up well. Pour the mixture over the potato slices. Toss the potato slices around to get them all coated. It’s easiest to just use your hands.

Spray a 9" diameter, non-stick cake pan liberally with Pam. Arranged a layer of potato slices around the pan. This will be the show side when you flip it out later, so try to make that first layer pretty while overlapping all the slices about halfway. Sprinkle on about one-third of the diced leeks and one-third of the grated cheese. You’ll end up with four layers of potatoes and three layers of filling. The last layer of potatoes will be bare on top so it will crisp up. You don’t have to use all the potatoes.

Bake at 450 for 65 minutes until fork tender. The potatoes on top will be pretty dark, but that’s ok, it’ll give it some texture. While baking, if the top potatoes start getting black, cover the pan with foil to finish the baking.

When they’re done, remove the pan from the oven and set it on the stove top. Use a butter knife to cut around the edges of the pan to loosen up any melted cheese crust. Place a cutting board on top of the pan, put on your oven mitts, and flip the board and pan over. Thump the bottom of the pan to attempt loosening up any sticky bits, then carefully lift off the pan. Make any necessary repairs if it didn’t come out clean to make a pretty presentation. Garnish it with some chopped parsley or chives.