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Pat steaks dry and sprinkle all over with salt and pepper. Heat oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then sear steaks in 2 batches, turning over once, until well browned on both sides, about 10 minutes total per batch for medium-rare. Transfer steaks as cooked to a platter.
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Add shallots to fat in skillet and sauté over moderately high heat, stirring occasionally, until softened and beginning to brown, 3 to 4 minutes. Remove from heat and add bourbon, then carefully ignite with kitchen match. Let flames burn out, then boil, stirring and scraping up brown bits, until bourbon is reduced to a glaze, 1 to 2 minutes. Add broth and boil until liquid is reduced by half, 3 to 4 minutes.
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Remove from heat and whisk in sage and mustard. Whisk in butter, 1 tablespoon at a time, until incorporated. Stir in any meat juices accumulated on platter and salt and pepper to taste.
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Slice steaks and serve with sauce.
