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1
Peel potatoes and cut into 1/4-inch-thick sticks. Rinse well, then soak in cold water, chilled, at least 2 hours.
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2
Drain potatoes and pat dry thoroughly. Heat 2 1/2 inches canola oil to 275u00b0F in a 4-to 5-quart heavy pot over medium heat.
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3
Fry potatoes in 2 batches, stirring once or twice, 3 minutes per batch (potatoes will not be golden), transferring with a slotted spoon to paper towels to drain. Return oil to 275u00b0F between batches.
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4
Heat oil to 320u00b0F. Refry potatoes in 2 batches, stirring once or twice, until golden and crisp, about 8 minutes per batch, transferring to fresh paper towels. Return oil to 320u00b0F between batches.
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5
Sprinkle hot fries with kosher salt and vinegar.
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6
Heat a 10-inch heavy skillet (preferably cast-iron; not nonstick) over medium-high heat until very hot. Pat steak dry and season with 3/4 teaspoon kosher salt and 1/2 teaspoon pepper.
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7
Add 1 1/2 teaspoons canola oil to skillet, then sear steak, turning once, 8 to 12 minutes total for medium-rare. During final minutes of cooking, add butter and thyme to skillet and baste steak with butter.
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8
Transfer steak to a cutting board and let rest 10 minutes.
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9
Add demi-glace to butter in skillet and simmer, scraping up brown bits, 30 seconds. Remove from heat and stir in olive oil. Cut meat from bone and slice. Serve steak with sauce spooned over it.
