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1
Position a rack in the center of the oven and preheat the oven to 350 degrees F. Lightly coat the inside of a 2- to 3-quart baking dish with the oil spray.
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2
Bring a large pot of lightly salted water to a boil over high heat.
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3
Add the cauliflower and return to a boil.
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4
Cook just until the cauliflower is barely tender, about 3 minutes (it will cook further during baking, so do not overcook).
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5
Using a scoop or a large slotted spoon, transfer the cauliflower to a large bowl of very cold water to cool.
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6
Drain well.
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7
Add the penne to the same pot of boiling water and cook just until barely tender (it will cook more in the oven), about 7 minutes.
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8
Drain well.
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9
Meanwhile, melt 4 tablespoons butter in a medium heavy-bottomed saucepan over medium-low heat.
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10
Whisk in the flour and let the mixture bubble without browning for 2 minutes.
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11
Whisk in the hot milk and bring to a boil over medium heat, whisking often.
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12
Return the heat to medium-low and let the sauce simmer until it is slightly thickened and has no raw flour taste, about 5 minutes.
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13
Stir in the Gruyere cheese.
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14
Season with salt and pepper.
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15
Transfer to the baking dish.
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16
Mix the Parmesan cheese and bread crumbs, sprinkle over the pasta, and dot with the remaining 1 tablespoon butter.
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17
Bake until the sauce is bubbling and the topping is golden brown, about 30 minutes.
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18
Let stand for 5 minutes, then serve hot.
