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1
Place potatoes in a saucepan and add broth to cover by about 1 inch.
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2
Season with 1/2 teaspoon salt and a pinch of pepper, or to taste.
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3
Bring to a boil and simmer until just tender but not falling apart, about 20 minutes.
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4
Remove potatoes from broth (reserve broth for another use) and allow to cool to room temperature.
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5
Meanwhile, in a medium skillet, combine onion with 1 tablespoon fat.
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6
Place over medium-low heat and saute until translucent, about 5 minutes.
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7
Season with salt and pepper to taste, and add 4 thyme sprigs.
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8
Reduce heat to low, and continue to cook, stirring occasionally, until onion is soft and caramelized, about 10 more minutes.
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9
Add vinegar, cook 1 minute more.
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10
Remove from heat, and discard thyme sprigs.
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11
Slice cooled potatoes into rounds 1/3 inch thick.
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12
Place a large cast-iron skillet over high heat, and add 2 tablespoons fat (or as needed to provide a thick coating on bottom of pan).
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13
When fat is extremely hot, add potatoes and allow to sit without stirring or shaking until seared and crispy.
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14
Turn potatoes and sear and crisp other sides.
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15
When well-browned, add caramelized onions, salt and pepper to taste, and stir to mix.
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16
Chop remaining 2 sprigs thyme (or if stems are woody, use leaves only), sprinkle over potatoes and serve.
