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1
In a 4-quart heavy kettle cover potatoes with salted cold water by 2 inches and simmer until tender, about 30 minutes.
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2
While potatoes are simmering, in a small saucepan heat milk with 6 tablespoons butter over moderate heat until butter is melted.
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3
Remove pan from heat and keep mixture warm, covered.
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4
Pit olives and separately chop olives and scallions.
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5
In a colander drain potatoes and when just cool enough to handle, peel.
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6
Coarsely smash potatoes with a potato masher and gently fold in milk mixture, olives, scallions, and salt and pepper to taste.
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7
Preheat oven to 400F.
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8
and butter a 3-quart gratin dish (about 15 by 10 by 2 inches).
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9
Mince garlic and chop parsley.
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10
In a nonstick skillet heat remaining 2 tablespoons butter over moderate heat until foam subsides and cook garlic, stirring, until fragrant, about 1 minute.
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11
Add bread crumbs and cook, stirring, until golden.
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12
Transfer mixture to a bowl and cool.
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13
Stir in parsley and Parmesan and season with salt and pepper.
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14
Cut tomatoes into 1/4-inch-thick slices.
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15
Spread potatoes in gratin dish and top with tomatoes, overlapping them to completely cover potatoes.
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16
Sprinkle crumb mixture over tomatoes and bake gratin in middle of oven until top is golden brown and tomatoes are tender, about 25 minutes.
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17
Gratin may be made 1 day ahead and cooled completely, uncovered, before being chilled, covered.
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18
Reheat gratin, uncovered.
