Brussels sprouts au gratin may be enough of a "labor of love" that you save them as a Thanksgiving side. But they're so delicious you may not be able to wait. Make your bechamel with gruyere and cheddar. Pour over thinly shaved Brussels sprouts, top with crispy panko, and bake until golden. Be sure to serve warm!
Heat the oven to 375°F with a rack positioned in the center.
Place a 2-quart saucepan over medium heat; add four tablespoons butter and heat until it melts, then foams. Sprinkle the flour over the butter, then work the flour into the butter. The mixture will be very thick and clumpy.
Gradually pour in the milk while whisking constantly. It will clump at first, then thin out. Maintain the heat and bring the mixture to a steady simmer; cook until the sauce is slightly thickened and easily coats the spoon, about 5 minutes.
Whisk in the salt, pepper and nutmeg. Reduce the heat to low, then add the cheeses in small batches (about ½ cup), whisking after each addition until very smooth. Turn off the burner, remove the pot from heat, cover, and set aside.
Heat a 12” ovenproof skillet over medium high heat; add the remaining 2 tablespoons of butter and heat until the butter melts, then foams.
Add the Brussels sprouts to the skillet along with a pinch of salt and toss to coat in the fat. Saute, stirring occasionally, for 5 minutes, until the Brussels sprouts start to soften and the volume decreases by at least a third. Add the shallots and garlic and saute, stirring frequently, for 2 minutes more.
If the bechamel sauce has started to clump, return to medium low heat and whisk quickly until smooth and pourable. Pour the sauce over the Brussels sprouts and stir until the skillet contents are well combined.
Combine the breadcrumbs and melted butter in a small bowl and mix until the breadcrumbs are evenly moistened. Sprinkle the mixture evenly over the gratin.
Transfer to the oven and bake uncovered for 25 minutes, until the sauce is bubbling around the edges and the topping is golden brown. Flip the oven to broil for 2-3 minutes to further crisp the topping, if desired.
Rest five minutes, then garnish with parsley and freshly grated nutmeg (if available). Serve hot or warm.