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1
Preheat oven to 350F.
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2
Rub 13x9x2-inch glass baking dish with cut side of garlic clove.
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3
Brush dish with olive oil.
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4
Heat 2 tablespoons oil in heavy large skillet over medium heat.
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5
Add onions, sprinkle with coarse salt, and saute until soft and beginning to brown, stirring frequently, about 13 minutes.
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6
Arrange 1/3 of potatoes in even layer in prepared dish.
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7
Sprinkle with coarse salt and pepper.
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8
Scatter half of onions over.
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9
Sprinkle with 2 tablespoons Pecorino Romano and 1 tablespoon capers.
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10
Repeat layering with half of remaining potatoes, coarse salt and pepper, remaining onions, 2 tablespoons Pecorino Romano, and 1 tablespoon capers.
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11
Drizzle with 1 tablespoon olive oil.
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12
Arrange remaining potato slices over.
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13
Sprinkle with coarse salt and pepper and remaining 1 tablespoon capers.
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14
Drizzle with remaining 1 tablespoon olive oil.
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15
Pour chicken broth over.
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16
Press down firmly on potatoes to compact gratin.
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17
Cover gratin tightly with foil and bake until potatoes are tender, about 1 hour 20 minutes.
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18
Uncover and sprinkle with remaining 4 tablespoons Pecorino Romano.
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19
Bake gratin uncovered until cheese is lightly browned, about 15 minutes.
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20
Let gratin stand 10 minutes at room temperature before serving.
