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1
In a heavy 5- to 6-quart kettle heat oil over moderate heat until hot but not smoking and cook onions with thyme and salt and pepper to taste, stirring occasionally, until golden, about 15 minutes.
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2
Reduce heat to moderately low and cook, stirring frequently to keep from scorching, until golden brown.
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3
Remove kettle from heat.
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4
While onions are cooking, bring a large saucepan of salted water to a boil for potatoes.
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5
Peel potatoes and cut crosswise into 1/4-inch-thick slices.
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6
Add potatoes to boiling water and cook 5 minutes from time water returns to a boil.
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7
Drain potatoes well in a colander.
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8
Preheat oven to 425u00b0F and lightly oil a 2- to 2 1/2-quart gratin dish or other shallow baking dish.
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9
Arrange half of potatoes, overlapping slightly, in dish and season generously with salt and pepper.
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10
Cover potatoes with onions, spreading evenly, and top with goat cheese.
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11
Arrange remaining potatoes, overlapping slightly, over cheese and season generously with salt and pepper.
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12
Dot gratin with butter.
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13
Gratin may be prepared up to this point 2 hours ahead and kept at room temperature, covered.
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14
Pour milk evenly over potatoes and bake 35 minutes, or until potatoes are tender and top is golden.
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15
Gratin may be made 1 day ahead and chilled, covered.
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16
Reheat gratin before serving.
