grease a small casserole or loaf pan with butter (I use a 8"x8" pyrex dish)
peel potato, slice 1/8" thick and layer in buttered pan - season each layer with salt, pepper, and sprinkle gruyere cheese
continue this process until you run out of potatoes or you've filled the dish/pan/casserole to an acceptable level (try not to go too close to the top, maybe 1/2 to 3/4 inch away)
combine milk & cream, whisk in nutmeg and poor over layered potatoes
cut piece of parchment to fit pan, butter parchment (if you want) and cover potatoes with parchment, pressing down a bit - cover again with foil
bake for 1 hour and check potatoes - if tender, uncover (parchment and foil - save the parchment) and bake 10 minutes to brown the top.
you can serve this immediately or weigh down and refrigerate for a few hours/overnight
to press: replace parchment on potatoes and weigh down with something of similar size that's heavy
reheat for 20 minutes at 350F
for a neat presentation, invert potatoes onto a baking sheet and slice into servings - but hey, do whatever you want.
you could use russet potatoes instead - when I do I slice into cold/acidulated water, shake off excess water and then build each layer - I think you'll get a little more bite to each layer with russets
I've done this recipe with 1.5 cups heavy cream or 1.5 cups of milk and mixes of both - I think any milk combo works well... its creamier with more cream, but the 100% milk version tastes just a great.
this recipe is still evolving - I've incorporated ideas from how2heroes.com, food52.com, greatbritishchefs.com, and a french recipe website that I've since misplaced - I was attempting to recreate a side dish I had from a great steakhouse in Madison NJ