Seriously. You’re allowed.
About the baking dish: My dish in the first photo is on the small side because I discovered that I’d only bought 2/3 of the potatoes I needed and scaled the recipe down. However, I found when I had the full amount of potatoes (retesting this) that it didn’t fit quite right in the 9-inch round baking dish Martha suggested (though it might for someone else). So, I’m going to make the somewhat annoying suggestion that you slice up all of your potatoes, grab a few dishes that may potentially work (a cast iron skillet would be great here), play around with the potato slices and when you figure out what dish you want to use, then put the butter/oil in the bottom and set it up. It sounds a bit insane but will ensure that your potato dish ends up looking exactly the way you want it to.
3 tablespoons salted or unsalted butter, melted
3 tablespoons extra-virgin olive oil
1/2 to 1 teaspoon red-pepper flakes (optional)(I used aleppo, which is milder and can therefore be used more generously)
4 pounds russet potatoes, peeled (smaller diameter potatoes are great, if you can find them)
4 shallots, peeled
8 sprigs thyme
Garnishes (optional): Bits of goat cheese, crumbles of bacon and/or bits of crisped pancetta
Preheat oven to 375 degrees. In a small bowl, combine butter and oil. With a sharp knife or manoline, slice potatoes crosswise very thinly. Figure out what baking dish you’d like to use [see Note above]; Martha suggests a 9-inch round baking dish (a deep dish pie pan would fit this bill) though I think you could go an inch bigger, an oval 1 1/2 to 2 quart casserole dish might also be pretty.
Once you’ve picked the dish that seems the best fit for your slices, pour a tablespoon or so of the butter/oil mixture in the bottom and spread it evenly. Sprinkle the oil mixture with a few pinches of coarse salt and red pepper flakes, if using; this will allow you to season both the top and underside of the potatoes. Arrange your potato slices vertically in the dish.
Thinly slice shallots with your mandoline and slide shallot slivers between potato wedges, distributing them as evenly as possible. Brush with remaining oil/butter mixture. Generously season your dish with salt; go easier on the red pepper flakes, if using. Bake 1 1/4 hours, then arrange thyme sprigs on top and bake until potatoes are cooked through with a crisped top, about 35 minutes more. If casserole seems to brown too fast, cover it with foil to slow it down. Add any garnishes, if using, and serve immediately.