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1
Preheat oven to 425F.
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2
Lightly butter a 2-quart gratin dish and dust with 2 tablespoons bread crumbs.
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3
Cut bok choy stems and center ribs into 1/2-inch pieces and coarsely chop leaves.
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4
Cook stems and ribs in a large pot of boiling salted water until just tender, about 5 minutes, then add leaves and cook 30 seconds.
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5
Drain in a colander and rinse under cold water until cool enough to handle.
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6
Squeeze out excess water by handfuls.
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7
Cook shallot in 1 tablespoon butter in a 12-inch heavy skillet over moderate heat, stirring, until softened, about 2 minutes.
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8
Add bok choy and cook, stirring, until greens are coated with butter and shallot, 1 to 2 minutes.
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9
Spread bok choy in baking dish.
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10
Melt 2 tablespoons butter in a 2-quart heavy saucepan over moderately low heat, then add flour and cook roux, stirring constantly, 2 minutes.
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11
Add milk in a slow stream, whisking constantly, and bring to a boil, whisking.
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12
Reduce heat and simmer, stirring, 5 minutes.
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13
Add nutmeg, salt, and pepper, then stir in Gruyere and 2 tablespoons parmesan and pour evenly over bok choy.
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14
Toss remaining 1/4 cup bread crumbs with remaining 2 tablespoons parmesan in a small bowl and blend in remaining 2 tablespoons butter with your fingertips until mixture resembles coarse meal.
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15
Season with salt and pepper.
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16
Sprinkle mixture evenly over gratin and bake in upper third of oven until bubbly and golden brown, about 20 minutes.
