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1
For the caramelized onions: Heat the oil over medium heat in a large nonstick skillet.
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2
Add the onions, vinegar, salt and pepper, stirring occasionally, until the onions are deep golden brown, about 20 minutes.
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3
Fries: Drain the potatoes and dry completely on a clean kitchen towel.
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4
In a large heavy-bottomed saucepan, pour enough oil to fill the pan about a third of the way.
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5
Heat over medium-high heat until a deep-frying thermometer inserted in the oil reaches 375 degrees F. (If you don't have a thermometer a cube of bread will brown in about 5 minutes.)
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6
While the oil is heating, in a large bowl, whisk together the flour, baking powder, baking soda, salt and 2 1/2 cups water until smooth.
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7
Coat the potatoes in the batter.
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8
Transfer 8 to 10 potatoes to the oil, allowing any excess batter to drip back into the bowl.
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9
Fry until golden and crisp, 5 to 6 minutes.
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10
Drain on paper towels.
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11
Repeat with the remaining potatoes.
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12
For the sauce: In a small skillet, melt the butter over low heat.
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13
Add the thyme and garlic and cook for 2 minutes.
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14
Pour the sauce over the fries and gently toss until coated.
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15
Place the fries on a platter and spoon the onions on top.
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16
Sprinkle with fontina and serve immediately.
