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1
Put oven rack in middle position and preheat oven to 425F.
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2
Brush a shallow baking pan (1/2 to 1 inch deep) with 2 tablespoons oil.
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3
Arrange tomato slices in 1 layer in baking pan and sprinkle with a scant 1/2 teaspoon salt and 1/4 teaspoon pepper.
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4
Roast tomatoes until just tender (not falling apart), about 20 minutes.
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5
While tomatoes roast, cut potatoes crosswise into 1/4-inch-thick slices and toss with 5 tablespoons oil in a large bowl.
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6
Remove tomatoes from oven and put oven racks in upper and lower thirds of oven.
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7
Arrange potato slices in 1 layer in 2 shallow baking pans (1/2 to 1 inch deep) and sprinkle each pan with a scant 1/2 teaspoon salt and 1/4 teaspoon pepper.
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8
Roast potatoes, switching position of pans halfway through roasting, until tender, about 16 minutes total.
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9
While potatoes roast, mince garlic and mash to a paste with a pinch of salt using a large heavy knife.
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10
Transfer paste to a bowl and stir in bread crumbs, basil, thyme, remaining 3 tablespoons oil, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon pepper.
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11
Preheat broiler.
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12
Arrange tomatoes and potatoes in 1 layer in gratin dish, alternating slices and overlapping them, and spoon any juices from tomatoes into dish.
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13
Sprinkle top with bread crumbs and broil 5 to 7 inches from heat, checking frequently after 1 minute (crumbs can brown quickly), until golden brown, about 2 minutes.
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14
Serve warm or at room temperature.
