Who doesn't love cheesy baked potato casseroles? This version spices things up a bit by using smoky chipotles in adobo sauce and fragrant Gruyere in the cheese sauce. Because a crunchy topping is just about mandatory, I sprinkle the whole thing with crispy panko bread crumbs. A mandoline will make quick work of slicing the potatoes.
Preheat the oven to 375 degrees F. Grease a large casserole dish with some butter.
In a saucepan, melt the butter over medium-high heat. Whisk in the flour to form a smooth paste. While whisking, slowly add the milk in a steady stream to form a smooth sauce. Bring the sauce to a simmer, whisking to prevent the formation of any lumps (be sure to get into the corners of the pan so nothing burns). Season with salt and pepper. Add the nutmeg and cheese, whisking until thick and smooth. Whisk in the pureed chipotles.
Put the potatoes in a large bowl and season with salt and pepper. Pour in the cheese sauce and toss to thoroughly coat the potatoes. Evenly arrange the potatoes in the greased casserole dish, sprinkle with the bread crumbs, and cover tightly with aluminum foil.
Bake until the potatoes are easily pierced with a knife, about 1 hour. Uncover and bake until the top is golden brown and crisp, about 15 minutes. Remove from the oven and let sit for 10 minutes before serving.
To make pureed chipotles, blend the entire contents of a 12-ounce can of chipotles in adobo sauce in a blender. What isn't needed can be jarred and refrigerated for up to 3 weeks.