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1
Preheat oven to 375F Use a little butter to grease a 2 quart casserole dish.
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2
Melt 3 tablespoons butter in a 4 quart dutch oven over medium heat.
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3
Add the onions, garlic, and a pinch of salt and cook until soft, about 15 minutes.
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4
Add the cream and bring to a simmer.
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5
Turn the heat to low and cook until the cream is slightly thickened, about 5 minutes.
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6
Meanwhile, thinly slice the brussels sprouts crosswise on a mandolin, slicing just until you get to the hard core.
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7
Melt another 2 tablespoons of the butter in a small skillet over medium heat.
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8
Stir in the flour and cook, stirring constantly, just until it begins to smell toasty, about 2 minutes.
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9
Mix the mustard with 1 tablespoon of water to make a thin paste.
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10
Whisk 2 tablespoons of the flour mixture into the onions along with the mustard paste, nutmeg, and 2 teaspoons salt; crank the heat up to medium and continue whisking until the mixture comes to a simmer and thickens, about 2 minutes.
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11
Cut heat to low and cook until the sauce loses any floury taste, about 10 minutes.
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12
Fit a food processor with the metal blade, and add the panko, Parmesan, 1 teaspoon salt, 1/2 teaspoon lemon zest, and the remaining 6 tablespoons butter.
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13
Process to a crumbly paste.
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14
Spread the crumb mixture on a sheet of parchment paper and top with another sheet.
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15
Use a rolling pin to roll the crust to fit the top of the casserole.
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16
Carefully pour the onion sauce in a blender and blend until smooth.
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17
Add the remaining zest from the lemon, all of the lemon's juice, and 1 teaspoon salt.
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18
Blend again until smooth.
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19
Pour the sauce back into the pot and fold in the sliced brussels sprouts.
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20
Bring the mixture to a gentle simmer and cook until the sprouts are wilted, about 5 minutes.
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21
Spoon the mixture into the prepared casserole and invert the prepared crust over the brussels sprouts.
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22
Bake until golden brown and bubbly, about 40 minutes.
