Preheat the oven to 325 degrees F. Two variations on this gratin could work: you could simply take your knife or mandolin and slice the potatoes into 1/8-inch thick rounds. Or you could do what's pictured above, which is to slice the tops and sides off. Halve the center piece of potato and then cut that into quarters. Cut each quarter into 1/8-inch thick slices and repeat the cutting process until you've gone through the remaining potatoes. Arrange the "domino" slices in a round 10-inch dish in a circular pattern, starting on the outside and working your way inward. Set aside.
In a medium saucepan, set over medium heat, melt the butter. Once the butter is melted, add the flour and mix until the mixture is smooth. Pour in the milk and stir until the mixture has reached a slight simmer and has thickened; the béchamel should be thick enough to coat the back of your wooden spoon. Add the cheese, pepper, thyme leaves and nutmeg. Mix until the cheese is all melty. Salt to taste. I added about 1/2 teaspoon. Fold in the chopped chantrelle mushrooms.
Pour the béchamel sauce over the arranged potatoes, using the back of your wooden spoon to even it out. Transfer the dish to the oven and bake for 25 to 30 minutes, until the potatoes are tender when poked with a knife and the sauce has bubbled. Remove and serve warm