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1
Preheat the oven to 350 degrees F. Spray a 13-by-9-inch baking dish with nonstick cooking spray.
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2
In a large skillet over medium-high heat, heat the olive oil.
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3
Add half of the sliced onions and cook 5 minutes, stirring frequently.
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4
Reduce the heat to medium.
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5
Cook until the onions are dark golden brown, 10 to 15 minutes, stirring frequently.
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6
Sprinkle with 1 1/2 tablespoons brown sugar and the cayenne.
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7
Remove with a slotted spoon and repeat the procedure with the remaining half of onions.
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8
Add more oil, if needed, and the remaining 1 1/2 tablespoons brown sugar.
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9
Drain well, and combine the onions, tossing well.
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10
In a small saucepan, melt the butter over medium heat.
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11
Sprinkle with the flour and stir well with a whisk.
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12
Cook 2 minutes, stirring constantly.
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13
Gradually whisk in the half-and-half, stirring constantly.
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14
Add in the mustard, paprika, pepper and salt.
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15
Cook, stirring frequently, until thickened.
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16
Remove from the heat.
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17
In the prepared baking dish, layer in the ingredients in this order: half of potatoes, half of sauce (about 1 cup sauce) and 1 cup Parmesan.
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18
Repeat with the remaining potatoes, cheese, sauce and top with the reserved onions.
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19
Cover with aluminum foil and bake until the potatoes are tender when pierced with a knife, about 30 minutes.
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20
Remove the foil and bake until bubbly and the cheese is melted, about more 15 minutes.
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21
Remove from the oven and let stand for 10 minutes before serving.
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22
Sprinkle with parsley.
